Sleep Physiology

Contrary to what some people may believe, sleep is not one homogenous state. In fact,
sleep is a progression through a series of states, each with its own unique characteristics.
In Sleep Physiology, let us take you on a journey, showing you just what goes on in
your mind and body once those eyes are closed tightly on your pillow.
Click on the clock to begin


This is a stage of very light sleep just as you start to drift off - a transition state between
wakefulness and sleep, if you will. During this stage, you are more likely to be awoken easily as
there is still a level of consciousness.

Non-rapid eye movement

In the first stage of non-REM sleep, you will lose the majority of your muscle tone - sometimes called
sleep paralysis. This means that your body will be unable to act out your forthcoming dreams. During
this stage, eye movements also become slower, as do brain waves, save for intermittent bursts of rapid
activity, called sleep spindles. This is your body preparing for deep sleep.

Non-rapid eye movement

As your body is now sufficiently prepared, the move to NREM2 marks the beginning deep sleep - sometimes
known as slow wave sleep. It is very difficult to rouse someone from this state of deep sleep, but if
you are woken, you'll feel especially drowsy and confused for a few minutes. This stage, however, is also
completely dreamless.

Non-rapid eye movement

Of all the stages of non-REM sleep, three is the deepest. This is also the stage which replenishes your
physical and mental energy the most. Therefore if you don't get enough of this stage of sleep, you won't
feel suitably refreshed in the morning. If you are known to sleepwalk or have night terrors, they also
occur during this period.

Rapid eye movement

As we get to the REM stage, so dreaming returns - as denoted by rapid eye movements. Interestingly if you
are awoken during this stage, when you fall back to sleep you will go straight back into it. REM sleep supports
the creation of long-term memories, and is also the period of the night where lucid dreams may occur.


Your night's sleep, however, does not consist of just passing through each of the stages described in sequence once. Instead, it works on a cycle, starting, aptly, at stage one. The cycle then progresses through stages two, three and four. At this point, stages three then two are repeated, before we fall into REM sleep, otherwise known as stage five. This whole cycle takes around 90 to 110 minutes, before we return to stage two and the cycle starts again.

Most adults will sleep for seven to eight hours per night, and will go through four to five cycles in this time.

Sleep Stages Through The Night




Immune System



Cerebral Cortex
Brain Stem
Corpus Callosum
Pituitary Gland

While we are awake, the body uses oxygen and food to produce energy - otherwise known as a catabolic state. In this state, more energy is used than saved, meaning the body's resources are being used up.

When we sleep, however, we move into an anabolic state, wherein conservation, repair and growth take precedence. Growth is stimulated by a protein hormone, with amino acids stimulating the repair and maintenance of muscles and bones. All the tissues in the body are renewed faster during sleep than at any time while awake.

We also produce melatonin to aid our sleep, secreted by the pineal gland in the brain. This controls our body's rhythms and sleep-wake cycles. As the body's temperature falls, levels of melatonin rise, helping to encourage a feeling of sleepiness.

Testosterone - otherwise known as the sex hormone - fertility hormones, luteinising hormones and follicle-stimulating hormones are also secreted during sleep.

It is generally accepted that increasing the amount of sleep you have while fighting an infectious disease or illness can aid your recovery. As we sleep, our body increases the production of certain proteins that fight disease, with production levels being much lower during wakefulness.

Sleep can also help to resist as well as cure infections too, as sleep deprivation can reduce the production of white blood cells - key components of the body's defence system.

The hugely important cancer-killer TNF is also active in our veins while we sleep, with research showing that people who stay awake till 3am have just one-third of the cells containing TNF than those who got to be at 11pm.

Our body and sleep cycles are also at the mercy of light and dark. Our in-built body clock - or circadian rhythm - is housed in the hypothalamus at the brain's base, and causes 24 fluctuations in the activity of our body's defence system.

This circadian rhythm helps to manage all the processes of our body, including everything from cell renewal to digestion. A network of chemical messages and nerves trigger our body rhythms, and are also controlled by our circadian rhythm. Maintaining regular levels of sleep ensures that our body can stick to a set rhythm, and feel suitably wakeful during the day and sleepy at night.

During the day, we constantly shed cells from the top layer of our skin. As we enter deep sleep, the metabolic rate of our skin speeds up rapidly and we experience increased production and reduced breakdown of proteins in the majority of the body's cells.

The term 'beauty sleep' rings true here, as the proteins produced during the night help to repair the damage caused by factors such as ultraviolet rays during the day. This is why sleeping during the day cannot be constituted as aiding beauty as we do not have the energy available to complete this task as it is otherwise engaged with other processes within the body.

Although we need saliva to help us eat while awake by lubricating the inside of the mouth, saliva flow is much reduced during sleep as it is not needed, hence why we experience the sensation of dry mouth in the morning.

This does not mean however that the mouth is inactive during sleep though. In fact, one in 20 adults will grind their teeth during their night unconsciously - otherwise known as bruxism. This will occur mainly between stages one and two of sleep, with poor alignment of the teeth in the jaw to blame.