Sleep is a critical component of life and survival for all living animals, especially humans. While we still don’t have an exact understanding, we know our bodies require significant periods of it on a regular basis to consolidate memories and learning, restore and rejuvenate energy systems, grow muscle and repair tissue, synthesize and balance hormones, and clear out waste products (i.e., metabolic toxins) from our brain via a lymphatic system that’s only activated during slow-wave sleep. In fact, during nighttime sleep our brain is very active. There’s almost as much neural activity during sleep as there is during waking periods.
During the night, our brain cycles through two major types of sleep. Non-REM sleep involves high-amplitude, low-frequency rhythms, whereas REM (rapid eye movement) sleep is characterized by low-amplitude, high-frequency EEG rhythms. There are four stages of non-REM sleep that occur before we reach the REM stage. The first state in a sleep cycle is light sleep (non-REM stage 1), followed by deeper sleep (non-REM stages 2-4), and a dream state referred to as REM sleep.
After the first REM stage is completed, we cycle back down through non-REM stages, 4, 3, and 2 before cycling back up through them again to REM. A full sleep cycle lasts about 90 minutes the first time through and is normally repeated several times each night, growing shorter each time. The last two sleep cycles of the night are usually alternations between stage 2 and REM sleep.
Brains that are deprived of REM sleep will subsequently produce more of it (i.e., REM “rebound”). It’s likely that each cycle of sleep has distinct neurorestorative processes.
Also relevant to our understanding of sleep are our circadian rhythms. These are cycles of sleep and wakefulness lasting about one day. Circadian rhythms occurring in an environment free of natural time cues (like if you lived in a dark cave) stabilize at a little over 24 hours. At any given moment, our degree of alertness depends in part where we are in our circadian rhythm. People fall somewhere on a continuum, with “morning people” being on one end and “evening people” being on the other end of that continuum, but this changes as we age. Young people tend to be “evening people” or have no preference; while older people (e.g., over 65) are “morning people.” There’s reason to believe that nocturnal lighting, especially the “blue” lights of computer screens and smart phones have a disruptive effect on our circadian rhythms.