It also is a complicated physiological phenomenon that scientists do not fully understand. According to the National Institute of Neurological Disorders and Stroke, about 40 million Americans suffer each year from some sort of chronic, long-term sleep disorder, and another 20 million people suffer occasional sleeping problems.
Insomnia is the most common sleep disorder.
It occurs more often among women and the elderly. The amount of sleep that a person needs to function normally depends on several factors such as age. Even though elderly adults need just as much sleep as young adults, as people age, they usually sleep for shorter amounts of time and they tend to spend less of their sleep time in the deep sleep stages. 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.