Experiencing sleep is relatively simple, without much thought required behind it. A mindful meal free of caffeine and sugar, avoidance of electronics, and a darkened environment provide the ingredients for a peaceful and restful slumber. The science of sleep, however, is more involved.
Why humans sleep has been the subject of numerous studies and speculation. The answer likely lies with what exactly happens when we sleep. Often in nature, form fits function. Hands allow for grabbing and climbing. Teeth take different shape in the mouth for tearing and chewing purposes. Likewise, sleep serves a purpose that explains why so many beings perform it.
Though a few varying degrees of sleepiness and wakefulness can be felt, true sleep itself is divided into two categories: non-rapid eye movement (NREM), and rapid eye movement (REM). Dreaming takes place in both stages; the main, easily visible distinguisher is the rapid movement of the eyes beneath the lids during REM sleep.
Approximately 75% of the night spent sleeping is NREM. NREM itself is broken into 4 stages:
- N1 (“Stage 1”) Light sleep, just dozing off
- N2 (“Stage 2”) Properly asleep; unaware of surroundings; body temperature drops, although heart rate and breathing remain unchanged
- N3 (“Stage 3” and “Stage 4”) Deep, restorative sleep; drop in muscle tension; energy recharged and tissue growth and repair is able to occur
Transference of short-term memories to long-term storage is also promoted by NREM sleep, and is dampened in its absence.
The remaining 25% of sleep is spent in REM sleep. Energy continues to be provided to the brain and body for the next day. The previous decrease in tension and breathing rate has leveled off at this point, and brain waves in REM sleep are similar to those when awake.
Dreams that occur during REM sleep are said to be based more heavily in emotions. That is why researchers believe that although NREM sleep is the most broad and helpful part of sleep, REM sleep in particular is important not only for continued energy restoration, but also to allow the mind to work through complex emotional situations.
A group of cells in the brain stem known as the subcoeruleus nucleus, moderate REM sleep and their injury or absence also means the absence of the characteristic muscle paralysis of sleeping.
Much of this activity can be traced through electrical signals given off in the brain. These include “sleep spindles” which, along with delta waves denote deep, restorative sleep.
Delta waves are recorded with a electroencephalogram (EEG), and are associated with Stage 3 of NREM sleep. Genetically speaking, those who produce stronger delta waves will sleep deeper; delta waves also influence the mounting need for sleep that builds up as the day progresses.
Sleep is a desire, just as food is. But the body has a way of retrieving sleep when it has been deprived for too long. Deep, NREM sleep, sometimes referred to as slow-wave sleep (SWS) is vital for recharging back to peak health to navigate the next day.
As mentioned, the brain is not inactive throughout the entirety of sleep, though sleep is important for having a clear head the next day. Rather, two parts of the brain are implicated in driving SWS. The ventrolateral preoptic nucleus in the hypothalamus and the parafacial zone in the brain stem each are involved in initiating deep sleep.
Many factors influence the quality of sleep felt, from environment, stress, biology, diet, and more. But the activities that occur during sleep remain consistent and are still the subject of much research today. By understanding the science of sleep, new ways to optimize the quality of that valuable rest can be elucidated as well.