What is sleep?

Sleep is defined as “a period of rest for the body and mind, during which volition and consciousness are in partial or complete abeyance and the bodily functions partially suspended. Sleep has also been described as a behavioural [sic] state marked by characteristic immobile posture and diminished but readily reversible sensitivity to external stimuli”.

What’s important is to understand that sleep is not simply a passive process that you sink in to at the end of the day, nor is it a complete blackout. As we said, we didn’t know anything more than the obvious until around 1953 when Eugine Aserinsky and Nathaniel Kleitmann decided to study an unusual fact about sleep: the twitching of the eyes which is observable in sleeping individuals. Their paper “Regularly Occurring Periods of Eye Motility, and Concomitant Phenomena, During Sleep” was the first to document what we now know as (thanks to the name given by these researchers) Rapid Eye Movement or REM sleep. This is heralded by many as the birth of modern sleep science – it was shown that sleep wasn’t passive or a state of complete unconsciousness which happens ‘to’ your brain, but rather a period during which your brain is active and which can only have been caused by your brain.

From there, more EEG studies have been done and so sleep has been relatively comprehensively mapped and described according to experimental findings. We now consider sleep to form 4 distinct stages spanning 3 stages of non-REM sleep and REM sleep itself. These are described below:

Alert awakeness

During the day your brain is, not surprisingly, at its most active. During times of extreme alertness such as when you are either hyper-aroused by an exciting event such as playing a sport or engaging in something dangerous, or when you are solving a very complex puzzle, your brain exhibits high frequency (25-50Hz), very low amplitude (0.5-2 µV) waves referred to as Gamma waves. These waves are associated with new experiences and learning, and it has been documented that individuals with learning disabilities display lower gamma frequency wave activity.

During less stressful periods of wakefulness, you will display 12-25Hz, 1-5 µV waves called Beta waves. These waves are involved with conscious thought, logical thinking and moderate to high amounts of cognitively-powered problem solving. Excessive beta wave activity is, conversely, associated with stress and anxiety. The higher your beta wave frequency goes, the higher your arousal levels become.

Relaxed wakefulness

It shouldn’t take too much explanation to make you aware that falling asleep very rarely happens suddenly in healthy people. We have phrases such as ‘winding down’ for the period of relaxation (or involuntary exhaustion) which occurs before sleep and this is displayed clearly on EEG readings. During the short time before you go to sleep you close your eyes and ‘switch your brain off’ to use another common phrase, and this induces Alpha wave activity of 8-12Hz and 20-80 µV. These are highly sensitive to eye opening and closing but can also be disturbed by starting to perform calculations or thinking too hard in general, which is something we will return to later in this module.

Falling asleep and sleep stage 1

Gradually as you relax more and more and your brain activity starts to drop further, you start to display 4-8Hz, 5-10 µV waves called theta waves and enter what is referred to as Stage 1 or non-REM stage 1 sleep, often called somnolence or drowsiness. Here your breathing starts to slow but muscles are still quite active so you can change your position and your eyes may open every now and then. To be in this state is to be able to hear conversations but to feel unwilling rather than unable to reply. It’s during this time that you may also experience hynagogic jerks, which are sudden awakenings accompanied by the feeling of falling. This short stage lasts around 10 minutes and you are very easily disturbed.

Note: It’s hard to tell exactly when you have fallen asleep because these things occur on a continuum of shifting frequencies and amplitudes and any distinctions we have described around them are necessarily arbitrary to a certain degree, despite being replicated across people.

Stage 2 sleep

This is the first stage of definite sleep. Your muscles relax almost completely, you become almost unaware of the outside world and you start to display only Theta wave activity. Interestingly, in stage 2 sleep you show two distinct phenomena:

• Sudden and short bursts of very high frequency (12-14Hz, like low frequency beta waves) referred to as ‘sleep spindles’. These are associated with activity in your thalamocortical system which is the system in which your thalamus (a region which receives sensory input) sends information to your cortex (the outer layer of your brain responsible for higher functions like memory, attention, consciousness and language). These are theorised to be critical for memory consolidation which we will come back to later
• K complexes which appear as a sudden large change in amplitude which have been shown to be absent in poor sleepers, and have been hypothesised to be part of a ‘sleep defence’ mechanism to keep you asleep despite environmental stimuli

Because of the process by which you sleep, as we will explain later, stage 2 sleep is the stage that makes up more of your sleeping time than any other stage, accounting for 45-50% of total sleep.

Stage 3 sleep

This is the stage referred to as deep sleep, and it is the time when your brain is the least active – producing slow delta waves of 0.5-4Hz but with a comparatively high 100-200μV (sometimes you will see it broken further down into stage 3 and 4). During this period your breathing, heart rate, blood pressure and body temperature drop to the lowest level during the night and you become almost completely unresponsive to outside stimuli. Deep sleep and delta waves are considered to be the most rejuvenating, restorative sleep as well as being crucial for memory consolidation – you will generally have 20-25% of your sleep being comprised of stage 3 sleep (most of this happens in the first half of the night) and a lack of delta wave sleep is what leads you to feel like you have had poor sleep quality.

On top of this, waking during delta wave sleep leads to sleep inertia, a groggy and confused feeling of near-drunkenness which can take up to 30 minutes to dissipate.

Stage 3 sleep is the period during which most sleep disturbances occur including bedwetting, sleep walking and talking, and night terrors which are described as overwhelming feelings of dread similar to a panic attack. Those who have night terrors will often sweat and show increased blood pressure and heart rate before waking. Waking someone having a night terror can cause them to lash out.