Sleep, 7(2): 155-167
© 1984 Raven Press, New York
The Effect of Sleep Before or After Learning
on Memory
Andrea Grosvenor and Leon C. Lack
Tke Flinders University of South Australia, Bedford Park, South Australia, Australia
Summary: Early studies in which it was found that learning followed by sleep
was better remembered than learning followed by wakefulness were interpreted as giving support for the Interference Theory of Forgetting. More recent
studies have shown better retention over the first half of the night's sleep
(slow-wave sleep) than over the second half (REM sleep), and conclusions
have been drawn that a Decay Theory of Forgetting is more strongly supported. Those studies, however, confounded the type of sleep following
learning with sleep prior to learning. When prior sleep was controlled in the
present study, there was no support for a first half-night sleep benefit, and,
contrary to Decay Theory, there was a second half-night benefit for high
imagery material. The strong detrimental effect of sleep prior to learning was
inconsistent with the Interference Theory of Forgetting and suggested, instead,
the importance of the consolidation process for long-term memory. Key
Words: Sleep stages-Long-term memory-Forgetting theories-Memory
consolidation.
In the present study we examined several variables which have, in the past, been
shown to mediate the effects of sleep on memory. These are the post-learning condition
(sleep or wakefulness), prior learning condition (sleep or wakefulness), physiological
arousal, length of the retention interval, and type of material learned.
Jenkins and Dallenbach (1) discovered that subjects who slept following learning
remembered more than subjects who were awake for the same length of time. They
used this finding as support for the Interference Theory of Forgetting, since it was
assumed that interference learning would be reduced during sleep. Although the beneficial effect of sleep on memory has subsequently been confirmed (2-4), the effect of
other variables on memory has raised some doubt regarding the Interference Theory.
The finding that sleep is most beneficial to memory when it follows learning immediately rather than later (5- 7) seems to implicate differential effects on a consolidation
process of memory. Studies that have manipulated the level of arousal directly (8,9)
Accepted for publication November 1983.
Address correspondence and reprint requests to Dr. Leon Lack, School of Social Sciences, Flinders
University, Bedford Park 5042, South Australia, Australia.
155
156
A. GROSVENOR AND L. C. LACK
or naturalistically using different times of the day (10-13) have shown that high arousal
at the time of learning is not usually beneficiai to immediate recall but dOes result in
better long-term retention than does low arousal. These results also seem to implicate
differences in a consolidation process following different levels of arousal during
learning.
The demonstration by Dement and Kleitman (14) that sleep can be divided into stages
with distinctive physiological characteristics provided a major impetus to current sleep
research, including the investigation of the relationship between sleep and memory.
The most distinctive stages are stage-4 sleep, characterized by high-amplitude, low.frequency EEG waves, and REM sleep, characterized by a desynchronized "activated" EEG and rapid conjugate eye movements. REM sleep appears to be closer to
wakefulness in terms of physiological arousal than does stage-4 sleep, which seems
most physiologically distinct from wakefulness. These differences in sleep stages may
be utilized to discriminate between theories of forgetting, since most stage-4 sleep
occurs in the first half of the night's sleep and most REM sleep occurs in the second
half (15). If learning is followed by the onset of a normal night's sleep and subjects are
awakened to test their memory after 4 h of sleep, the retention interval will differ
markedly from that of subjects awakened after 4 h of sleep to learn the material and
allowed 4 h more sleep before testing. Using this method, Yaroush et al. (16), Barrett
and Ekstrand (17), and Fowler et al. (18) found that learning followed by the first half
of the night's sleep led to much better memory of neutral material than did learning
followed by the second half ofthe night's sleep. The second-half condition was superior
to a retention interval of wakefulness. These experimenters concluded that stage-4
sleep following learning was more beneficial to memory than REM sleep following
learning. Ekstrand et al. (19) argued that this difference more strongly supports the
Decay Theory than the Interference Theory of Forgetting since interfering learning
should be absent in both stages of sleep, but the decay of memory should be more
rapid when physiological arousal is higher, in REM sleep.
However, in all these studies retention over the second half-night's sleep may have
been detrimentally affected by the 4 h of sleep immediately prior to learning. Stones
(20), Shearer (21), and Ekstrand et al. (19) have demonstrated that sleep prior to
learning has a detrimental effect on long-term recall, although it has no effect on rate
of learning or immediate recall of items. Ekstrand et al. (19) found the detrimental
effect for prior sleep with sleep lengths from 0.5 h to as long as 6 h of prior sleep.
Thus, the 4 h of sleep prior to learning in the second half-night condition would have
detrimentally affected recall. Although Ekstrand et al. (19) suggested from indirect
evidence that the prior sleep effect was not as strong as the difference between first
half-night and second half-night retention, they add" ... Nevertheless, the difference
in magnitude of the two effects is not great and it could very well be that if there were
a way to eliminate the confounding from the prior sleep, we would find no difference
between the first and second half of the night (no difference between REM and stage4 sleep." (p. 437). The confounding of a prior sleep effect with the comparison of the
second half-night and first half-night sleep was also noted more recently by Idzikowski
(22), who concluded, " ... It is difficult to assess Ekstrand's experiments, as most of
his subjects and especially his 'second half of the night' subjects will undoubtedly have
been suffering from the effects of 'prior sleep' ." (p. 314).
What is needed in the experimental design to assess the strength of the prior sleep
effect is a fourth group that is awake before and after learning. This would complete
Sleep. Vol. 7, No.2. 1984
SLEEP AND MEMORY
157
a 2 x 2 design such that sleep or wakefulness prior to learning can be compared with
sleep or wakefulness following learning. Then, only if the difference in retention between the first and second half-night groups is larger than the prior sleep effect can a
first half-night advantage be demonstrated to exist. Its statistical verification would be
tested as the interaction between the main effects of prior learning condition (sleep or
wakefulness) and post-learning condition (sleep or wakefulness).
Although arousal at the time of learning affects long-term recall, Ekstrand et al. (19)
discounted the idea that differences between prior sleep and prior awake groups could
be due to differences in arousal level, since all subjects were fully awake, had performed an arousing warm-up task, and showed no differences in speed of learning.
However, as mentioned earlier, a lower level of arousal does not necessarily result in
a slower rate of learning. Although the prior sleep subjects may have been fully awake
at the time of learning, physiological indices of arousal, such as body temperature,
may still have been low and may have been a factor in long-term retention. The present
study included the physiological arousal indicators of oral temperature, heart rate, and
blood pressure.
Research into the effects of sleep on memory has also shown differences in recall
due to the type of material to be remembered. Grieser et al. (23) found that REMdeprived subjects recalled more "non-threatening" items than did non-REM-awakened
subjects, whereas non-REM-awakened subjects remembered more "threatening"
items than REM-deprived subjects. Tilley and Empson (24) found that the recall accuracy of stories was significantly poorer when followed by REM deprivation than
when followed by stage-4 deprivation. In addition, there was less forgetting during
REM recovery sleep than during stage-4 recovery sleep. The results of animal studies
have shown that REM deprivation leads to poor recall of learned avoidance behavior
(25). In studies in which slow-wave sleep has been assumed to be more beneficial to
memory than REM sleep, neutral material such as nonsense syllables, digits, or word
pairs has been used (e.g., Refs. 16-18). This suggests that REM sleep is important for
memory of high association value and emotive material, whereas slow-wave sleep
benefits retention of neutral material.
In one experiment Fowler et al. (18) varied the imagery value of word pairs in a
paired-associate learning task with the retention interval consisting of the first or
second half of the night's sleep or wakefulness. They found a large effect of imagery
on memory and a large effect of the retention interval condition, but no interaction.
However, there may have been an interaction over longer retention intervals. Kleinsmith and Kaplan (26) found that paired responses to high arousal stimulus words were
remembered poorly when retention was tested immediately after learning, but remembered well by groups tested over longer intervals. These results have been replicated
by Kleinsmith and Kaplan (27), Walker and Tarte (28), and Butter (29). Corteen (30)
found a positive correlation between evoked arousal of words to be learned and recall
after no delay, 20 min, or 2 weeks, with the size of the correlation increasing with
longer retention intervals. The present study included paired associates with stimulus
words of high or low imagery value and response words of high or low imagery value,
and extended the design of Fowler et al. (18) by including a long-term retention interval
of 6 days in addition to the initial test 4 h after learning. The interaction of word
imagery and type of retention interval, which Fowler et al. (18) did not find with a 4h retention interval, may emerge over the longer retention interval of 6 days. If there
is a first half-night sleep benefit for low imagery words, there may not be that advantage
Sleep. Vol. 7. No.2. 1984
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A. GROSVENOR AND L. C. LACK
for high imagery words, which should benefit more from REM sleep, particularly when
retention is tested after the longer interval in the present study.
METHODS
SUbjects
The subjects were 40 (19 female and 21 male) volunteers, who were students at
Flinders University. The subjects were divided randomly into four groups, each containing 10 subjects, with an attempt made to keep the numbers of males and females
balanced.
Each group represented one of the four possible combinations of two different conditions (sleep or awake) prior to learning and post-learning. They were designated as
the Awake-Learning-Sleep group (ALS) , the Sleep-Learning-Sleep group (SLS), the
Sleep-Learning-Awake group (SLA), and the Awake-Learning-Awake group (ALA).
All subjects were naive to the purposes of the experiment. Two subjects who failed to
reach the performance criterion during the learning task were discarded and replaced
by two others.
Learning material
The learning material consisted of 16 pairs of nouns with frequencies of at least 50
per million (31). Four of these pairs were made up of high imagery stimulus and high
imagery response words; four pairs were of high imagery stimulus and low imagery
response words; four pairs were of low imagery stimulus and high imagery response
words; and four pairs were of low imagery stimulus and low imagery response words.
High imagery words were those having an imagery rating of at least 6.5, and low
imagery words were those with an imagery rating of no more than 3.43 from the imagery
scale of Paivio et al. (32). Each pair consisted of words with no association rating in
the tables compiled by Postman and Keppel (33).
The words, printed in block letters, were recorded on video tape to provide study
and test trials. In the study trials the pairs were presented for 3 s each, with 1 s between
each pair. In the test trials the stimulus words were presented for 3 s each, with 3 s
between each word for responding. A total of 10 study trials alternating with 10 test
trials, each with a different random sequence of word pairs or stimulus words, respectively, were tape recorded. During the experiment the learning material was presented on a National video monitor with a 25-cm screen.
The laboratory
The laboratory consisted of an experimenter's room and three sleep rooms connected
by a corridor. The sleep rooms were carpeted, air-conditioned, and sound-attenuated;
each contained a bed and a chair. The experimenter's room contained a table, six
chairs, and the video equipment.
Procedure
The subjects were asked to try to maintain their normal sleep/waking pattern for 2
days prior to the experiment, not to sleep during the day of the experiment, and not
to consume any alcohol or caffeine following their evening meal that night. On arrival
at the laboratory at 2200 h the subjects were asked if they had followed these instructions.
There were usually four or five subjects participating in the experiment on anyone
night. At about 2230 h oral temperature, heart rate, and blood pressure were measured
Sleep, Vol. 7, No.2, 1984
159
SLEEP AND MEMORY
")
TABLE 1. Experimental design
Treatment conditions and times
Treatment group
I.
2.
3.
4.
ALS
SLS
SLA
ALA
Prior
learning
2300-0230
Awake
Sleeping
Sleeping
Awake
0245-0315
Postlearning
0330-0730
Learn
Learn
Learn
Learn
Sleeping
Sleeping
Awake
Awake
0745-0800
6 Days
later
1030-1045
Test
Test
Test
Test
Retest
Retest
Retest
Retest
for all subjects. They were then randomly assigned to groups. The experimental design
is summarized in Table 1. At about 2300 h the subjects in the SLS and SLA groups
went to bed, while subjects in the ALS and ALA groups stayed in the experimenter's
room. Awake subjects spent the time in the experimenter's room talking, playing board
games or cards, reading, or studying, and had physiological measures taken hourly.
No stimulants, depressants, or other drugs were permitted during the experiment.
At 0230 h subjects in the SLS and SLA groups were roused and joined the others
in the experimenter's room. Physiological measures were taken for all subjects. They
were then seated around the table so that they all could see the video monitor clearly.
The mean time for the start of learning was 0245 h. The learning instructions were read
and explained. The subjects learned the paired-associate list by the study-test method
until the criterion of 12 out of 16 correct responses in one test trial was reached.
Physiological measures were taken when the subjects completed the learning task, and
then those in the SLS and ALS groups went to bed at a mean time of 0325 h. Those
in the ALA and SLA groups remained in the experimenter's room for the rest of the
night, and their physiological measures were taken hOUrly.
At 0730 h subjects in the ALS and SLS groups were awakened and joined the others
in the experimenter's room. Physiological measurements were then taken on all subjects. A single, paced recall test trial was given. Following this a matching test was
given in which response words in a randomly arranged list had to be matched to the
appropriate stimulus words. The purpose of this was to include a test of recognition
memory.
Six days after this test the subjects returned to the laboratory and at a mean time
of 1035 h were unexpectedly given a paced recall retest followed by a matching retest
in which the order of the stimulus and response words was randomly rearranged from
that of the initial test.
RESULTS
Prior and post-learning condition effects
Two-way Analyses of Variance (ANOVA) were carried out with prior condition
(sleep or awake before learning) and post-learning condition (sleep or awake in the 4
h following learning) as the independent variables. As illustrated in Fig. 1, the initial
degree of learning was comparable for all groups. No significant (p < 0.05) effects of
prior or post condition were found for the number of words correct on the criterion
trial. Furthermore, the absence of significant effects in the number of trials to the
criterion indicated that all groups had comparable rates of learning.
Sleep, Vol. 7, No.2, 1984
A. GROSVENOR AND L. C. LACK
160
• AWAKE - SLEEP
• SLEEP - SLEEP
16
o
SLEEP - AWAKE
o
AWAKE - AWAKE
12
FIG.!. Mean for the four groups of the
number of words correctly recalled in a
test trial (out of 16) on the criterion
learning trial (at least 12 correct), on the
initial paced recall 4 h after learning and
recall retest 6 days after learning.
CRITERION
TRIAL
INITIAL
TEST
RE-TEST
To compensate for any individual differences in the number of words correct on the
criterion learning trials, the percent loss from criterion trial to recall trial was calculated
for each subject as the amount of material forgotten. Table 2 shows the mean percent
loss from the number of words correct on the criterion trials to the initial paced recall
test. The ANOVA on these data showed a significant main effect for the prior condition
[F(l,36) = 14.77, p < 0.01] in which prior awake groups (circles in Fig. 1) recalled
better than prior sleep groups (squares). There was also a significant effect of postlearning condition [F(1,36) = 5.08, p < 0.05] in which "post" sleep groups (filled
symbols) recalled better than "post" awake groups (open symbols). There was no
significant interaction [F(1,36) = 1.912, p > 0.05], between prior and post-learning
conditions. There were no significant condition effects for performance on the initial
TABLE 2. Mean percent loss from the criterion trial to
the initial paced recall test 4 h after learning and to the
retest of paced recall 6 days after learning, and mean
percentage correct on the matching retest
Group
Memory test
ALA
SLA
ALS
SLS
4-h Recall loss
6-Day recall loss
Mean matching score
6.4
25.6
34.4
55.1
73.8
0.8
23.3
85.0
14.6
59.2
73.1
Sleep, Vol. 7, No.2, 1984
85.6
SLEEP AND MEMOR Y
161
matching test, but the very high scores for all groups (mean = 90% correct) on this
test indicated the presence of a ceiling effect.
Mean recall performance for the four groups 6 days after learning is illustrated in
Fig. 1. The percent loss from the criterion trial to the recall retest (see Table 2) showed
only a significant main effect of prior condition: F(l,36) = 20.47, P < 0.01. Neither
the post-learning condition (F < 1.0) nor .the interaction (F < 1.0) was significant. On
the matching retest scores, prior condition was also a significant main effect: F(1,31)
= 4.66, p < 0.05. Neither the post condition (F < 1.0) nor the interaction (F < 1.0)
was significant in the matching retest.
In summary, sleep following learning benefited recall performance only over the
short interval (4 h), but not in long-term (6 days) recall or recognition. On the other
hand, sleep prior to learning, although it did not affect the rate of learning, did result
in more forgetting over the short and long periods of retention. The absence of significant interactions between prior and post-learning conditions suggested that where
there was a benefit of post-learning sleep, the ALS group (first half-night sleep) compared with its appropriate control group, ALA, was not benefited more than the SLS
group (second half-night sleep) in comparison with its control group, SLA. In other
words, there was no evidence of an advantage of first half-night sleep over second
half-night sleep when prior condition was controlled.
Word imagery effects
Although word imagery value of stimulus words and response words had an effect
on the number of correct responses on the criterion trial, the focus of interest in the
present study was the differential forgetting between groups and between word pair
types from criterion to test trials. Stimulus word imagery and response word imagery
had no significant effect on the number of words lost from the criterion trial to the
initial paced-recall test or the retest of recall. However, in the matching retest 6 days
after learning, low imagery response words were correctly matched significantly less
well (F(1,144) = 59.41, p < 0.01] than were high imagery response words. Since the
initial matching test 4 h after learning showed generally high performance (mean =
90%) and no differences between the groups or word pair types, the subsequent differences after 6 days suggested a greater amount of forgetting of low imagery response
word pairs, at least in recognition memory. This was confirmed in a direct test of the
difference between high and low imagery response words in the decrease of correct
matches from the initial test to the retest session: t(34) = 2.66, p = 0.012. Also in the
matching retest, a significant prior-condition effect [F(1,144) = 10.21, P < 0.01] and
interaction of prior condition by response word imagery [F(1,144) = 5.49, p < 0.05]
suggested that prior sleep was particularly detrimental to long-term recognition of
paired associates with low imagery response words. These effects are illustrated in
Fig. 2.
Because response word imagery seems to determine retention, two-way ANOVAs
between the groups were carried out separately for the high imagery response word
pairs and for the low imagery response pairs. With high imagery response pairs, analysis of the number of words lost from the criterion trial to the initial paced test showed
only a significant main effect of prior condition: F(1,36) = 6.42, p < 0.05. The number
of words lost from the criterion trial to the paced retest, however, showed significant
effects for prior condition [F(1,31) = 67.83, p < 0.01] and a significant interaction of
prior and post-learning conditions [F(1,31) = 6.05, p < 0.05], as illustrated in Fig. 3.
Sleep, Vol. 7, No.2, 1984
A. GROSVENOR AND L. C. LACK
162
10°1
90
o
PRIOR AWAKE
\:::::::::::::\ PRIOR SLEEP
FIG. 2. Mean percentage correctly
matched by the prior learning awake
groups (ALA and ALS) and also by the
prior learning sleep groups (SLA and
SLS) analyzed separately for the high
imagery response words and low imagery response words in the matching retest 6 days after learning.
This interaction indicated a smaller difference in forgetting between the post-learning
sleep groups than in the post-learning awake groups. If the difference between the
post-learning awake groups (ALA and SLA) is taken as the baseline measurement of
the effect of prior condition, then the smaller difference between the post-learning
sleep groups (ALS and SLS) would suggest that second half of the night sleep had a
beneficial effect and/or the first half of the night sleep had a detrimental effect on longterm recall of high imagery response words. Scores on the matching retest showed
only a significant effect of prior condition: F(1,31) = 7.49, p < 0.05.
o
Z
lJ.J
t-
PRIOR AWAKE
h:,,:}::) PRIOR
SLEEP
O
~
f2
~
a:
~
FIG. 3. Mean for the four groups of the
number of high imagery response words forgotten (out of 8) in the paced recall retest 6
days after learning,
u..
o
a:
lJ.J
ID
~l
z
z
«
lJ.J
~ O.:'--~:-:-'-~
ALA
SLA
AWAKE
ALS
SLS
SLEEP
POST LEARNING CONDITION
Sleep. Vol. 7, No.2, 1984
SLEEP AND MEMORY
163
With low imagery response pairs the analysis of the number of words lost from the
criterion trial to the initial paced test showed significant main effects for both prior
condition [F(1,36) = 29.43, P < 0.01] and post-learning condition [F(1,36) = 14.3, P
< 0.01], but no interaction between these main effects. However, in the long term, as
measured by the paced recall retest and matching retest, there were no differences
between the groups in the amount of forgetting of low imagery response pairs.
Physiological measures
Heart rate and blood pressure measures showed no clear patterns or differences
throughout the night. For all groups combined, oral temperature dropped significantly
[t(39) = 6.79, p < 0.01] from a maximum mean of 36.93°C at 2230 h to a minimum
mean of 36.47°C at 0230 h. Temperature was increased after learning [t(39) = 3.89, p
< 0.01] and then showed no significant change throughout the rest of the night. Comparing the prior learning conditions, the prior sleep groups (SLA and SLS) showed a
greater drop in oral temperature from 2230 h to the time of learning (0230 h) [t(38) =
2.29, p < 0.05] and had a lower mean oral temperature at 0230 h [t(38) = 2.08, p <
0.05] than did the prior awake groups (ALA and ALS).
The relationship between oral temperature and performance measures was examined
more directly with Pearson Product-Moment correlation coefficients. Although there
was no difference in the number of trials to the criterion between prior sleep and prior
awake groups, within the prior sleep groups only there was a significant correlation
between the drop in oral temperature to the time of learning and the number of trials
to criterion [r(18) = +0.422, P < 0.05]. However, all other correlations of drop in oral
temperature or temperature at the time of learning with measures of retention were
not significant (p < 0.05).
DISCUSSION
Because EEG recording was not taken on the subjects in the sleep condition of the
present study, there was no objective verification of the presence, extent, or type of
sleep in the present study. However, the procedures used in the present study were
identical to those used by Barrett and Ekstrand (17), who confirmed the prevalence of
slow-wave sleep in the first half of the night's sleep, whether it occurred before or
after learning, and of REM sleep in the second half-night condition. Without the slight
discomfort of EEG electrodes, and with university students who tend to have relatively
short sleep latencies (34), it seems reasonable to assume that sleep prevailed in the
sleep conditions of the present study. This was, in fact, reported by all subjects in the
sleep conditions.
The detrimental effect of prior sleep on memory is consistent with earlier findings
(19-21). In this study we found that prior sleep still has a strong effect after a retention
interval of 6 days when tested by either recall or recognition. It was suggested earlier
that the prior sleep effect may be due to lower arousal at the time of learning despite
the lack of a difference in rate of learning. It has been shown that sleep does reduce
physiological arousal (35). Consistent with this finding, the prior sleep groups (SLA
and SLS) showed a greater drop of oral temperature from 2230 h to 0230 h than did
the prior awake groups (ALA and ALS), and thus had lower mean temperature at the
time of learning. However, when individual drops of oral temperature before learning,
Sleep, Vol. 7, No.2, 1984
164
A. GROSVENOR AND L. C. LACK
or absolute temperatures at the time of learning, were correlated with subsequent
memory loss, there were no significant correlations-for all groups combined, or the
prior sleep or prior awake groups separately. Therefore, although prior sleep reduces
oral temperature and retention, there was no direct relation between oral temperature
and retention. Either oral temperature is not an accurate indicator of arousal, or arousal
is not a very useful explanation for the prior sleep effect in the present study.
The post-learning sleep benefit in the present study is consistent with past findings
and replicates the study of Barrett and Ekstrand (17), in which the time-of-day factor
was also controlled by setting the retention interval for all groups from 0250 h to 0650
h. However, the post-sleep benefit over the first 4 h of retention was not strong [F(1,36)
= 5.08, p < 0.05] and was not apparent over 6 days retention in either recall or
recognition tests of memory. This finding contradicts those of Graves (5) and Richardson and Gough (6), who found that post-learning sleep did not lead to better retention over intervals of less than 48 h, but did for greater retention intervals. Because
the time-of-day effect was not controlled in those studies, it is difficult to evaluate their
finding of a delayed beneficial effect of post-learning sleep. We have concluded from
the present study that the benefit of sleep compared with wakefulness following
learning is neither great nor long-lasting.
Consistent with Fowler et al. (18), there was no interaction between the type of postlearning condition and stimulus word imagery or response word imagery over the 4-h
retention interval. Because the post-learning benefit did not remain over the 6-day
retention interval, the lack of a significant interaction with stimulus or response word
imagery over this longer interval was not surprising. The prior learning condition effect
was generally stronger than that of the post -learning condition and was also manifested
after the 6-day retention interval. Although there was no interaction with stimulus or
response word imagery after the 4-h retention interval, there was an interaction between the prior learning condition and response word imagery in the matching retest.
Prior sleep was particularly detrimental to the long-term retention of low imagery
response words. Baddeley (36) suggests that high imagery words are easy to learn in
part because they have both a verbal and a visual code. It was assumed that pairs
containing high imagery words, particularly high imagery response words, would lend
themselves to the mnemonic device of visual image formation to learn the paired
associate, whereas low imagery pairs would more likely be learned by rote memorization. Most of the subjects reported this to be the case. It is also a plausible explanation for the better retention, over the long-term interval, of the high imagery response
words.
When performance was analyzed separately for low imagery response word pairs,
it was found that they were affected detrimentally by prior sleep and beneficially by
post-learning sleep over the 4-h retention interval, but not over the 6-day interval. This
result could be due to a floor effect, since recall of low imagery response pairs was
very poor for all groups in the retest. The absence of a prior and post-learning condition
interaction did not support a first half-night advantage with the low imagery response
words. Therefore, even with material that would be considered relatively neutral, there
appears to be no advantage of slow-wave as compared with REM sleep in the retention
interval.
High imagery response pairs showed a detrimental effect of prior sleep in recall tests
at both retention intervals and in the longer interval recognition test. The interactions
Sleep, Vol. 7, No.2, 1984
SLEEP AND MEMORY
165
between prior and post-learning conditions in the recall retest suggest that the detrimental effect of prior sleep was reduced when learning was followed by sleep in the
second half of the night. Therefore, when prior learning condition is controlled, the
second half of the night's sleep after learning appears to be beneficial to long-term
recall of material that could be considered more emotive, or having a greater number
of associations. This is consistent with the notion that REM sleep benefits the consolidation of emotive material (23), high association value material (24), or material
that calls more for divergent processing (37). Alternatively, the different sleep stages
may differentially affect the retention of mnemonically learned material but not rotelearned material. An obvious suggestion from the present study would be to investigate
whether the benefit of the second half-night's sleep occurs for emotive and high imagery material per se or whether the effect is due to the mnemonic method of learning.
Ekstrand et al. (19) interpreted- their results and the earlier ones of Ekstrand (2)
Fowler et al. (18), and Yaroush et al. (16) as supporting a first half-night sleep advantage
over a second half-night sleep retention interval. Because slow-wave sleep predominates in the first half of the night and REM sleep predominates in the second half, they
suggested that these results supported a Decay Theory of Forgetting based on the
assumption of more rapid decay of memory in REM sleep than in slow-wave sleep.
However, when the confounding effect of prior sleep was controlled in the present
study, the absence of an interaction between the post-learning and prior learning conditions for all learning material combined suggested that there was no difference in the
type of sleep following learning. Therefore, with regard to the effect of sleep following
learning, the present findings do not support a Decay Theory, but they are consistent
with the Interference Theory of Forgetting. The disappearance of a significant postlearning sleep advantage over the 6-day interval may reflect the proportionally decreasing size of the interference-free interval after learning as compared with the size
of the retention interval.
However, it should be kept in mind when considering theories of forgetting that the
strongest effect in the present study is the prior sleep effect. It was stronger than the
post-learning sleep effect over the 4-h retention interval, and it was still present (p <
0.01) after the 6-day retention interval, when the post-learning effect was no longer
present. Decay Theory can be neither supported nor rejected by the prior sleep effect,
since memory decay is conceived as a process following learning. Interference theory
would also be considered unrelated to the prior sleep effect if it is assumed that interference learning can only follow the original learning. However, it is well known that
proactive interference as well as retroactive interference affects memory (38,39). Proactive interference learning should be reduced by sleep and result in a benefit of prior
sleep. Thus the strong detrimental effect of sleep prior to learning can only disconfirm
the Interference Theory of Forgetting.
The results of the present study seem to support most strongly a memory Consolidation Theory. Consolidation is drastically reduced for material presented during sleep,
particularly slow-wave sleep (40-42). Whether the reason for this is the presence of a
biochemical substance, such as growth hormone, as suggested by Ekstrand et al. (19)
and Hoddes (43), or a decrease in some aspect of physiological arousal, it seems
plausible that the effect may still be present to some extent after waking, thus impairing
the consolidation in long-term memory of material learned at this time. The presence
of a post-learning sleep benefit is also consistent with a Consolidation Theory. In
Sleep. Vol_ 7. No.2. 1984
166
A. GROSVENOR AND L. C. LACK
particular, the benefit of the second half-night's sleep for retention of high imagery
response words is inconsistent w~ith Decay Theory and Interference Learning Theory,
but is consistent with the findings of experimenters (23,24) who have suggested that
REM sleep plays an active role in memory consolidation of vivid and emotive material.
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