Sleep has been a puzzling thing for researchers to fully understand. While it is obvious that sleep is necessary for everyone to survive and that it plays key roles in body and brain function, its exact effects are not totally understood. Sleep is important in mood, emotional response, and cognitive function.
Sleep is broken into stages, 1, 2, 3 and rapid eye movement (REM) sleep. The first stage, stage 1, acts as a transition period between wakefulness and sleep. It only occurs for short periods of time (usually 5-10 minutes). It is characterized by slow brain waves called theta waves. In stage 2 the brain begins rhythmic wave patterns called sleep spindles. In this stage the body also begins to react and lower metabolisms such as body temperature and heart rate. Stage 3 (used to be broken into two separate stages) is characterized by delta waves, which are slow waves. This is the stage of deep sleep or slow wave sleep (SWS) as it is called. The last stage is REM sleep in which the brain increases significantly in activity and is characterized by eye movement. This is the stage where dreams generally occur due to the increased brain activity. While the brain activity has a sharp increase, other voluntary muscles relax and essentially become paralyzed which is thought to keep people from acting out their dreams.
Sleep stages generally tend to occur in cycles and will go down from 1 to 3 and then back up before REM sleep, which occurs on the edge of sleep and wakefulness. A full cycle usually occurs about every 90 minutes and repeats throughout the night.
links to pictures of sleep stage cycle and brain wave activity
Sleep patterns also occur on a larger level such as biological clocks. An individual’s biological clock is factored by rhythms that fall into two main categories, exogenous or endogenous. Exogenous rhythms come from external stimuli such as environmental cues (sunlight). Endogenous rhythms come from internal sources in an organism. Things such as body temperature oscillations are endogenous. Other factors that influence the circadian sleep rhythms are hormone release. Melatonin is the post prevalent. Its release triggers sleep and can shift circadian rhythms when administered. It is controlled by the SCN and acts on a feedback loop back to the SCN to regulate its release.
While the exact function for sleep is not known there are several hypothesis that could explain the reason for sleep. These include energy conservation since metabolism lowers during sleep, memory consolidation, protection from predators due to inactivity during certain hours of the day, recovery of body systems, and emotional discharge to allow lower activity in emotional pathways to restore them.
Whatever the case it can be seen that sleep is needed from sleep deprivation studies in which subjects show irritability, paranoia, decreased cognitive function, and even hallucinations.