There is no definitive answer as to why people sleep, although scientists agree that sleep is necessary for performing cognitive skills, such as learning and language. One theory for why people need sleep is that it gives the body a chance to repair itself from the day's activities.
Lack of sleep causes several problems with the brain's ability to function. After one night without sleep, a person can become grumpy, groggy, irritable or forgetful. Concentration becomes more difficult, and the attention span shortens considerably. If lack of sleep persists, the parts of the brain that control memory, language and planning become severely affected. Research shows that sleep deprivation can be similar to drunkenness in its effect on a person's ability to respond to changing situations and to make rational judgements.
These problems are rectified by sleeping. When a person sleeps, they go through cycles of sleep lasting 90 to 110 minutes, with four main stages and a rapid eye movement stage, known as REM. Stages one through four include light sleep, true sleep and the deep sleep of stages three and four. During these stages, the body's breathing and heart rate gradually slow as the brain's activity increases, culminating in the brain's production of delta waves. In the REM stage the brain's activity increases to levels often higher than waking levels, and breathing and heart rate increase. This is the stage in which dreams occur. After REM sleep, the cycle begins again, repeating about five times a night.