Sleep disorders

Sleep disorders involve any difficulties related to sleeping, including difficulty falling or staying asleep, falling asleep at inappropriate times, excessive total sleep time, or abnormal behaviors associated with sleep.
Causes, incidence, and risk factors
More than 100 different disorders of sleeping and waking have been identified. They can be grouped in four main categories:
Problems with falling and staying asleep
Problems with staying awake
Problems with adhering to a regular sleep schedule
Sleep-disruptive behaviors
PROBLEMS WITH FALLING AND STAYING ASLEEP
Insomnia includes any combination of difficulty with falling asleep, staying asleep, intermittent wakefulness and early-morning awakening. Episodes may be transient (come and go), short-term (lasting as long as 2 to 3 weeks), or chronic (long-lasting).
Common factors associated with insomnia include:
Physical illness
Depression
Anxiety or stress
Poor sleeping environment such as excessive noise or light
Caffeine
Alcohol or other drugs
Use of certain medications
Heavy smoking
Physical discomfort
Daytime napping
Counterproductive sleep habits:
Early bedtimes
Excessive time spent awake in bed
Disorders include:
Psychophysiological insomnia (learned insomnia)
Delayed sleep phase syndrome -- where a patient's internal clock is constantly out of synch with the "accepted" day / night phases; for example, patients feel best if they can sleep from 4AM to noon
Hypnotic-dependent sleep disorder -- insomnia resulting from building tolerance to, or stopping, certain types of sleep medications
Stimulant-dependent sleep disorder -- insomnia resulting depending on, or stopping, certain types of stimulants