Most scientists agree there are four stages of sleep, with the fourth being rapid eye movement (REM). This is the phase where people dream; the phase gets its name from the way the person’s eyes move rapidly under their eyelids. The other three phases move the brain from conscious to unconscious, as seen by the way the brainwaves move through the different phases. As the brainwaves slow, the body relaxes and the body temperatures drops. The third phase of sleep has the body entering a deep sleep, which is what helps us feel refreshed. This stage is when the body is least active, with breathing, brain activity, and heart rate being significantly reduced. REM begins about 90 minutes after a person falls asleep, and it lasts about 10 minutes during the first cycles. With each progressive sleep cycle, REM gets longer. The body becomes more active, and the brain starts to be nearly as active as when a person is awake. As a person ages, the less REM sleep they get.
Everyone has their own unique circadian rhythm (circadian is Latin for “around day”). The circadian rhythms helps to determine when a person needs to sleep and for how long. Everyone’s circadian rhythm is established partly by their genetics, and helps to establish their biological clock. The controls to the biological clock are stored in a brain structure called the suprachiasmatic nucleus. A person’s sleep-wake homeostasis plays a role in the sleep cycle. Ironically, homeostatic sleep is directly linked to the amount of time a person is conscious.
Sleep is at least as essential as other biological requirements like eating and drinking. Based on sleep studies, if a person is unable to get solid deep sleep, their ability is directly affected, with a person being less able to learn. During sleep, the brain’s synapses begin to repair themselves after being active all day. Another study monitored sleep spindles (spikes in the brain’s activities during sleep). This study showed that a person listening to sounds during REM would be able to identify those sounds when awake, but not if they heard the sounds during the other stages of sleep. Researchers were surprised to learn that people who heard the sounds during the non-REM phases also had a harder time learning those sounds when awake compared to completely new noises. This indicates that the brain is actively suppressing information to allow the synapses to properly rest.
A third study seems to have verified both of these studies, showing that sleep improves learning, helps the brain to relax and repair itself, and helps the person to stabilize.
For more details and detailed findings, check out What Happens in the Brain When We Sleep?