What is sleep, anyway?

March 14, 2019

While sleep is a regular part of everyone’s lives (at least we hope it is), have you ever taken time to think about what happens to you when you sleep?

There are two phases of sleep: Rapid eye movement (REM) and non-rapid eye movement (non-REM). The various brain activities underway during different stages of sleep are as distinct from each other as they are from wakefulness, and each of them serve a critical purpose.

Non-REM sleep consists of four stages, which range from light dozing to deep sleep. Throughout this state of sleep, muscle activity is still functional, breathing is low, and brain activity is minimal. Approximately 75 percent of the sleep cycle is spent in non-REM sleep. Simple thought processes may be reported if you are awakened in any stage of non-REM sleep, however you will likely not recall any specific dream. The non-REM “deep sleep” stage in the sleep cycle is absolutely essential for our body to regenerate and recharge. Key processes such as repair and growth of muscle and tissues, immunity, and energy production all require quality deep sleep to take place.

REM sleep typically occupies 20% – 25% of total sleep among adults, so during a normal night’s sleep we usually experience about four or five periods of REM sleep. These periods are short at the beginning of the night and longer toward the end. During REM, the activity of our brain’s neurons is similar to that during waking hours. REM sleep is physiologically different from the phases of non-REM sleep. Vividly recalled dreams mostly occur during REM sleep, as do nocturnal penile tumescence (NPT).

While non-REM sleep recharges our body, nightly REM sleep recharges our mind by strengthening neural pathways, especially those related to memory, and boosts our brain’s supply of key mood-balancing neurotransmitters, such as serotonin and dopamine.