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. Sleep-wake Homeostasis – Regulates circulating levels of neurotransmitters responsible for drowsiness (adenosine) and alertness (serotonin and norepinephrine).
  2. 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 physiology
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

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.

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.

sleep physiology
Source: Nat Rev Nephrol © 2009 Nature Publishing Group

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 release of the hormone cortisol, increasing alertness and energy, and inhibiting sleep. Under normal 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.

sleep physiology
Source: Sleep Sync © 2015

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.

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 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. Harvard Medical School – Harvard Health Publications – http://www.health.harvard.edu/staying-healthy/repaying-your-sleep-debt
  2. Harvard Medical School – Harvard Health Publications – http://www.health.harvard.edu/newsletter_article/Sleep-and-mental-health
  3. National Institute of Neurological Disorders and Stroke – Understanding Sleep – http://www.ninds.nih.gov/disorders/brain_basics/understanding_sleep.htm
  4. Sleep-wake cycle: It’s physiology and impact on health – National Sleep Foundation 2006 – https://sleepfoundation.org/sites/default/files/SleepWakeCycle.pdf
  5. How sleep works – http://www.howsleepworks.com/how_circadian.html