Sleep comes in two varieties, slow-wave sleep (or NREM sleep) and REM (Rapid Eye Movement) sleep.
REM sleep is the stage in which dreaming most often occurs.
REM sleep is often called paradoxical sleep. This is because the brain waves of a sleeping brain are very similar to those of an awake brain.
Our bodies are paralysed during REM sleep.
Dolphins sleep with only half of their brain to prevent drowning. Once one side is fully rested, it flips over to the other half.
In 1964 an American high school student named Randy Gardner stayed awake for 11 days and 25 minutes. He is still the current world record holder, due to the Guinness Book of World Records withdrawing sleep deprivation from inclusion due to health risks.
Sleep before learning increases our ability to make new memories, in other words, learn more stuff.1
Sleep after learning aids memory consolidation.1
Sleep helps us to de-clutter our brain by jettisoning unwanted and overlapping copies of information.
Even mild sleep deprivation can cause micro-sleeps, where we enter a sleep state for only a few seconds.
Four hours of sleep a night for ten consecutive nights is the equivalent of going without sleep for 48 hours.1
People need around 8 hours of sleep every night in order to function adequately.
Sleep patterns vary both between people and across the lifespan. Teenagers, for example, have a circadian rhythm that makes them more alert later in the day but difficult to rouse in the morning, while older people often sleep less at night but nap during the day.
1 Matthew Walker (2017), Why We Sleep Penguin.