About half of all adults over the age of 65 have some sort of sleep disorder, though it is not clear if this is a normal part of aging or a result of medications that older people commonly use. Falling asleep and waking up are governed by various chemical changes in the brain and blood. Thus foods and medicines that affect the balance of these various chemicals also affect how easily we can fall asleep and how well we sleep. Smoking and alcohol can also cause a loss of deep sleep. Both REM and deep sleep are essential parts of the normal sleep cycle.
Sleep is a dynamic process, during which the brain is very active.
There are recognized stages of sleep, each of which is characterized by a different type of brain wave activity. A complete sleep cycle, from the beginning of stage 1 to the end of REM usually takes about an hour and a half. Stage 1 is a light sleep during which the muscles begin to relax and a person can be easily awakened. During stage 2, brain activity slows down and eye movement stops. Stages 3 and 4 comprise deep sleep, during which all eye and muscle movement ceases.