Sleep is a critical component to the life and survival of all living animals, especially humans. Although we still do not have an exact understanding, we do know that our bodies require long periods of it on a regular basis to enhance memories and learning, restore and replenish energy systems, develop muscles and Repair tissues, synthesize and balance hormones, and remove waste products (such as metabolic toxins) from our brain via the lymphatic system that is only activated during slow wave sleep. In fact, while we sleep at night, our brain is very active. There is almost as much neural activity during sleep as during periods of wakefulness.
During the night, our brain goes through two main types of sleep. Non-REM sleep involves high-amplitude and low-frequency rhythms, while REM sleep is characterized by low-amplitude and high-frequency EEG rhythms. There are four stages of non-REM sleep that occur before we reach the REM stage. The first state in the sleep cycle is light sleep (non-REM phase 1), followed by deeper sleep (non-REM phases 2-4), and the dream state is referred to as REM sleep.
After the first REM stage is complete, we go back down through the non-REM stages 4, 3, and 2 before cycling back through them back into the REM. A complete sleep cycle takes about 90 minutes the first time and usually repeats several times each night, getting shorter each time. The last two sleep cycles at night are usually an alternation between stage 2 and REM sleep.
Brains that have been deprived of REM sleep will produce more of it later (that is, REM reflux). It is possible that each sleep cycle has its own distinct neural regeneration processes.
Also relevant to our understanding of sleep are our circadian rhythms. These are cycles of sleep and wakefulness that last about one day. Circadian rhythms that occur in an environment devoid of natural time signals (such as if you live in a dark cave) stabilize for just over 24 hours. At any given moment, our degree of alertness depends in part on our place in our circadian rhythm. People fall somewhere on a continuum, with ‘morning people’ at one end and ‘evening people’ at the other end of this continuum, but this changes as we age. Young men tend to be “nightlife” or have no preference; Whereas older people (for example, those over 65 years old) are “morning people”. There is reason to believe that night lighting, especially the “blue” lights of computer and smartphone screens, has a devastating effect on our circadian rhythms.