Sleep can be objectively studied by using polysomnography, a technique combining the measurement of brain activity (electroencephalography or EEG), eye movements (electro-occulography), and muscle tone (electromyography). Using this technique, two types of dormancy can be distinguished: non-rapid-eye-movement (NREM) and rapid-eye-movement (REM) sleep, which are organized in a series of rest cycles during the night.

NREM Sleep

Different brain wave patterns detected during NREM sleep can be subdivided into four distinct sleep stages, simply labeled stages 1, 2, 3, and 4.

From a state of drowsiness, the individual slips into stage 1, then progresses sequentially through the other stages of NREM sleep. Of short duration (about 5 minutes), stage 1 is a transitional phase between wakefulness and more definite rest. During this light sleep, the arousal threshold is low and the brain wave signal is characterized by low-amplitude and high frequency waves. Progressively, the amplitude of the signal increases and its frequency decreases as the individual enters subsequent NREM stages. Stage 2 lasts 10 to 15 minutes and, for most people, corresponds to the phenomenological experience of falling asleep. Stages 3 and 4 are considered the deepest stages of slumber and together last between 20 to 40 minutes in the first rest cycle.

They are often referred to as “delta,” or “slow-wave sleep” because of the presence of slow EEG waves of high amplitude called delta waves. After reaching stage 4, the EEG pattern reverses through stage 3, stage 2, and finally gives place to the first REM sleep episode.


In REM sleep, the EEG pattern is very similar to the one observed in stage 1.

Brain waves of low-amplitude and high frequency are, however, accompanied by rapid movements of the eyes under the lids. The REM stage is often referred to as “paradoxical sleep” because it is characterized by a loss of core muscle tone while the activity in the brain and in the autonomic system are at a level similar to that seen during wakefulness.

Apart from occasional muscle twitches, the body is essentially paralyzed in this stage. The most vivid dreams occur during REM sleep, even though non-narrative dream-like activity may also be recalled when subjects are woken up from the NREM stages. Even when dreams are not remembered, REM episodes and dream activity nonetheless occur during a normal night of slumber.

Healthy Sleep

In healthy young adults who follow a regular sleep schedule, the proportion of time spent in REM sleep is about25%, while the remaining 75% is spent in NREM sleep. NREM Stage 1 represents about 5%, Stage 2 another 50%, and Stages 3–4 about 20%. The distribution of sleep stages follows a highly structured and well-organized cyclic pattern, with slow-wave sleep occurring mainly in the first third of the night and REM sleep becoming more prominent and more intense in the latter part of the night and early morning hours.

The proportion of time spent in each stage of slumber, as well as sleep quality and quantity, can be altered by several factors, most notably, advancing age.

Circadian and Homeostatic Factors

The propensity to sleep and the type of sleep experienced are highly dependent on circadian and homeostatic factors. Sleep is just one of many biological functions (e.g., body temperature, melatonin and growth hormone secretion) that are regulated by circadian rhythms. A small brain structure located in the hypothalamus, serves as a biological clock to regulate this alternation between different states while interacting closely with time cues provided by the environment. The light-dark cycle is the most important of these cues.

Social interactions, work schedules, and meal times are other extrinsic time cues that also contribute to regulating our sleepwake cycles. Homeostatic factors can also impact significantly on sleep. For instance, the time to fall asleep is inversely related to the duration of the previous period of wakefulness.

With prolonged sleep deprivation, there is an increasing drive to sleep. Upon recovery, there is a rebound effect producing a shorter slumber latency, increased total sleep time, and a larger proportion of slow-wave dormancy. Following sleep loss, there is a preferential recovery of slow-wave sleep, followed by REM sleep. Daily variations in core body temperature, which are also controlled by circadian factors, are closely tied to sleep-wake patterns.

At its lowest point in the early hours of the day (e.g., 3:00 to 5:00 a.m.), body temperature starts to rise near the time of awakening and peaks in the evening. Alertness is at its maximum during the ascending slope of the body temperature curve. In contrast, sleepiness and dormancy itself occur as temperature decreases. In the absence of time cues or any schedule constraint, individuals tend to choose a bedtime that is closely linked to a decrease in body temperature, while awakening occurs shortly after it begins to rise again. There is a slight drop in temperature in mid-afternoon, which can be associated with a temporary decline in alertness and a prime time for falling asleep. In our next article, we will mention a few techniques and adjustments that can be used to promote sleep based on these scientific facts.