The human body goes through five stages of sleep when we lay down to rest, and interference with the sleep cycle can cause tiredness and irritability the following day. Science has discovered two distinct types of sleep, known as rapid eye movement (REM) sleep and non-REM sleep. The latter type of sleep has four different stages:

Stage 1: non-REM sleep. This sleep occurs in the first moments after you have laid your head down on your pillow and closed your eyes. The eyes move slowly and muscle movement ceases. In this stage, the sleeper can be easily awakened by noise or other disturbances as she drifts in and out of sleep.

Stage 2: non-REM sleep. In this stage of sleep, the person is actually asleep and the sleeper is not aware of her surroundings. Body temperature drops, breathing and heart rate are regular, and eye movements decrease significantly or are non-existent. Brain waves slow down, though there may be bursts of activity.

Stage 3: non-REM sleep. This is deep sleep, characterized by even slower brain waves and less sporadic bursts of brain wave activity. Breathing slows and muscles relax. Sleepers are hard to awaken during this stage. Children sometimes wet the bed during this stage of sleep.

Stage 4: non-REM sleep. This is the deepest sleep and is characterized by very slow brain waves and no sporadic bursts of brain wave activity. As with Stage 3 non-REM sleep, sleepers are hard to wake up. Scientists believe that tissue repair takes place during this stage of sleep. Also, hormones may be released to assist with growth development.

Stage 5: REM sleep. The REM stage is the sleep during which we dream. It is characterized by rapid eye movements even though the eyes are closed. Breathing is rapid, irregular, and shallow. The heart rate and blood pressure increase. Arms and leg muscles experience a type of paralysis that keeps people from acting out their dreams.

The process of dreaming is not well understood but the process does stimulate the parts of the brain used for learning and memory. Some studies suggest that dreams are a way for the brain to sort and store information gathered throughout the day, rather like a filing cabinet.

This last cycle of the stages of sleep occurs about an hour to 90 minutes into the sleep ses

Sleep cycles through all of the stages until the sleeper awakes. However, the length of time of each cycle changes throughout the night, with Stages 3 and 4 non-REM sleep becoming shorter in each cycle as sleep progresses. As a result, most sleep is spent in Stages 1 and 2 non-REM sleep.

The amount of a sleep a person needs, as well as the time spent in stages of sleep, varies by individual, but generally adults need seven to eight hours of rest within a 24-hour period. Otherwise, sleep deprivation can result in lower productivity and health concerns.

sion.