Sleep: What is it, and Why do we need it?

Although we view sleep as an escape from the activity of the day, sleep is a fairly active process itself. Throughout a healthy night's sleep, structures in the hypothalamus and brain stem cycle repeatedly through five stages characterized by different brain waves produced during the cycles. The five cycles can be broken up into two main categories: REM (rapid eye movement) and non-REM (slow wave) sleep cycles (Waxman, 2003). This web page will focus on REM and non-REM sleep, but for more information on the specifics of the four sleep cycles, visit http://www.medicinenet.com/sleep/article.htm.
REM sleep, the most crucial part of the sleep cycle (studies have shown that rats deprived of REM sleep have prolonged learning and memory difficulties (Kim et al., 2005), occurs 70 to 90 minutes after falling asleep. REM sleep is initiated by dorsal midbrain and pontine tegmentum neurons. During the REM cycle, sometimes referred to paradoxical sleep, the locus ceruleus releases norepinephrene, a neurotransmitter in the brain that affects food intake, levels of arousal, and mood (Kalat, 2004). The brain then produces delta waves, waves nearly identical to ones observed when a person is awake (Figure 2). Along with a shift in brain waves, often heart rate and blood pressure increase, intense visual imageries (dreams) occur, and muscles can often become temporarily paralyzed. Narcoleptics still experience REM sleep, but instead of entering sleep at stage one and progressing to REM sleep, they often instantly enter REM sleep upon falling asleep. This sudden loss of consciousness and muscle control endangers narcoleptics and sometimes those around them (MedicineNet, 2005).
Figure 2. Brain waves collected by an electroencephalograph (EEG). Notice the similarity between the awake, alert waves and REM sleep waves. Permission pending, http://sun.science.wayne.edu/~bio340/StudentPages/Soliman/physweb.html
As stated previously, the hypothalamus is a structure in the brain that regulates the onset of sleep stages. Within the hypothalamus, hypocretins reside in cells and possibly aid in regulating the neurons involved in REM sleep. Some researchers believe that when Hcrtr2 is stimulated in humans with normal sleep patterns, hypocretins promote wakefulness and repress REM sleep. However, in narcoleptics, a mutation in Hcrtr2 disables this pathway, allowing the sudden onset of REM sleep (http://www.healthgene.com/canine/C110.asp).