The circadian rhythm is the pattern of fluctuations in bodily processes that occur regularly each day. Circadian rhythms are found in mostly all species. These rhythms are synchronized with the 24 hour cycle of day and night. In humans, the sleep-wake cycle operates on a circadian rhythm that is close to 24 hours in length. For instance one's body temperature is not maintained at a constant 98.5 degrees Fahrenheit. It follows a circadian rhythm in which the body's temperature falls a few degrees during the middle of the night, rises early in the morning, and then peaks by mid-day. .
Stage 1 sleep, brain waves become small and irregular with varying frequencies. You can easily be awakened during this stage and may not even realize that you were sleeping. During Stage 2, sleep begins about 2 minutes after Stage 1 sleep. It is characterized by bursts of brain wave activity that are represented by spindle-shaped waves called sleep spindles. You spend more than half your sleep time in Stage 2 sleep. This is a deeper stage of sleep, but you still can be readily awakened. Stages 3 and 4 of sleep (also called delta sleep or slow-wave sleep), are characterized the appearance of large, slow brain waves. This is the period of deep sleep, is when it becomes difficult to get one to wake up. The distinction between Stage 3 and Stage 4 is the proportion of the large, slow brain waves. In Stage 3, the long, slow brain waves constitute 50% or fewer of the brain wave patterns. In Stage 4, they are more than 50%. These stages are known as the NREM sleep stages.
After the NREM stages, one enters the REM sleep. REM is the stage where one's eyes dart around under the closed eye lids. Also, REM is the stage of sleep most closely associated with dreaming. The brain becomes more active during REM sleep, which is why it is sometimes called active sleep. Brain wave patterns during REM sleep are very similar to those during the states of being awake.