WHAT HAPPENS WHEN WE SLEEP
The average person will spend 1/3 of their life asleep. We all need sleep, it is a vital part of our existence, and
without it our health, emotional wellbeing and cognitive performance suffers. So, what actually happens
physiologically while we are sleeping?
WHAT CAUSES US TO SLEEP AND WAKE UP?
The sleep-wake cycle is responsible for making us sleep at night and for keeping us awake during the day. It is
controlled by two physiological systems that work together and balance each other:
1.
2.
Sleep-wake Homeostasis – Regulates circulating levels of neurotransmitters responsible for
drowsiness (adenosine) and alertness (serotonin and norepinephrine).
Circadian Rhythms – Control the secretion of sleep stimulating hormones (melatonin) and sleep
inhibiting hormones (cortisol).
Sleep-Wake Homeostasis
From the time that we wake up, the homeostatic drive for sleep accumulates, reaching its maximum in the late
evening and making us fall asleep. Neurotransmitters circulating in the brain are one of the physiological
mechanisms responsible for this sleep need. Neurotransmitters are nerve signalling chemicals which influence
brain cell (neuron) function by either inhibiting or stimulating different parts of the brain. Neurotransmitters
such as serotonin and norepinephrine (stress hormone) keep some parts of the brain active while we are awake.
Over the course of the day another chemical called adenosine continuously rises, causing drowsiness and the
increased need to sleep the longer we are awake. Adenosine also has the effect of inhibiting serotonin and
norepinephrine, making us less alert and helping us to fall asleep. Adenosine gradually breaks down while we
sleep, helping us to wake up feeling alert and well rested in the morning.
Commonly used stimulants, like the caffeine in coffee, tea, cola and energy drinks (as well as the theophylline in
tea and chocolate), work as adenosine receptor blockers, inhibiting or dampening its sleepiness effect, and
thereby maintaining alertness.
Sleep need represents Sleep-wake
Homeostasis.
Sleep urge represents the Circadian Rhythm.
Post-lunch dip occurs at around 2-3pm in the
afternoon.
Source: University of Sheffield
Circadian Rhythms refer to the cyclical changes—like fluctuations in body temperature, hormone levels, and
sleep—that occur over a 24- hour period. Most circadian rhythms are controlled by the body’s biological “clock”,
located in the brain, which is scientifically known as the suprachiasmatic nucleus or SCN. This body clock controls
sleep by regulating the secretion of sleep stimulating hormones, such as melatonin, and wakefulness promoting
hormones, such as cortisol.
© 2016 The Centre for Personal Performance. All rights reserved.
www.personalperformance.com.au
Melatonin is responsible for making us drowsy and is sometimes called the Dracula hormone as levels increase
at night when it is dark, and are inhibited by light. Melatonin levels typically begin to rise at around 8-9pm at
night, reaching its peak at around 2-3am and are at the lowest levels by around 7-8am in the morning.
In the morning, after waking, we are generally
exposed to light. Natural or artificial light reaches
photoreceptors in the eye creating signals that travel
along the optic nerve to the SCN. The SCN then sends
signals to several brain regions, including the pineal
gland, which responds to these light-induced signals
by switching off the production of melatonin.
Light signals that reach the SCN also stimulate the
Source: Nat Rev Nephrol
release of the hormone cortisol, increasing alertness
© 2009 Nature
and energy, and inhibiting sleep. Under normal
Publishing Group
circumstances, cortisol should naturally decrease the
longer we are awake (except at times where there is
chronic or increased episodic levels of stress), reaching its lowest levels in the evening therefore allowing the
body to wind down, helping to promote sleep. Light and darkness act as external cues that “reset” the biological
clock every day and help to regulate feelings of wakefulness and sleepiness.
The cells in our eyes contain a unique light-sensitive pigment, that is most sensitive to short wavelength “blue
light”. By working/reading on a screen after 9pm, the blue light emitted from that screen (which is usually
situated relatively close to the eye) halts the production of melatonin and stimulates the release of cortisol which
can in turn impact the body’s ability to fall asleep and to get into deeper more restorative stages of sleep.
STAGES OF SLEEP
There are two forms of sleep – REM sleep and non-REM sleep. These different types of sleep are distinguished
by brain-wave activity and the presence or absence of rapid eye movement (REM). During non-REM sleep, body
temperature drops, muscles relax, and heart rate and breathing slow. During REM sleep dreams occur, and body
temperature, blood pressure, heart rate, and
breathing increase.
There are 4 stages that occur during non-REM
sleep progressing from light sleep (stage 1 & 2)
to the deepest stages of sleep (stage 3 & 4). A
typical night's sleep consists of four or five
REM/non-REM cycles with occasional, brief
episodes of wakefulness. Most Stage 4 sleep
occurs during the first few hours of sleep with
REM sleep occurring more frequently in the
second half of sleep.
Source: Sleep Sync © 2015
Stage 4 of non-REM sleep is characterised by slow brain waves and is the deepest phase of sleep. Deep sleep is
essential for physiological recovery and restoration. Stage 4 of non-REM sleep produces physiological changes
that boost immune system functioning, and release growth hormone - stimulating protein synthesis, tissue
growth and repair. Most of stage 4 sleep only occurs during the first few hours of sleep which is the period of
© 2016 The Centre for Personal Performance. All rights reserved.
www.personalperformance.com.au
sleep most likely to be disrupted by alcohol consumption and stimulating activities before bed (screen time,
caffeine etc). Good sleep hygiene habits are therefore essential to ensure you don’t miss out on the benefits of
deep sleep for recovery and optimal physiological functioning.
Studies have found that REM sleep is essential for cognitive performance – enhancing learning and memory, and
contributing to emotional health.
The underlying mechanisms involved in sleep are still being studied, but there is significant evidence indicating
that sleep is essential for optimal health and cognitive function. With sleep deprivation associated with chronic
stress, impaired mental acuity, depression, obesity, and increased risk of diabetes and cardiovascular disease.
WANT TO KNOW MORE?
For more information on how sleep effects your health, resilience to stress and mental acuity, and for tips to
improve your sleep click here.
References
1.
2.
3.
4.
5.
Harvard Medical School – Harvard Health Publications - http://www.health.harvard.edu/staying-healthy/repaying-your-sleepdebt
Harvard Medical School – Harvard Health Publications - http://www.health.harvard.edu/newsletter_article/Sleep-and-mentalhealth
National Institute of Neurological Disorders and Stroke – Understanding Sleep http://www.ninds.nih.gov/disorders/brain_basics/understanding_sleep.htm
Sleep-wake cycle: It’s physiology and impact on health – National Sleep Foundation 2006 https://sleepfoundation.org/sites/default/files/SleepWakeCycle.pdf
How sleep works - http://www.howsleepworks.com/how_circadian.html
© 2016 The Centre for Personal Performance. All rights reserved.
www.personalperformance.com.au