Activity monitoring has a long history both in medicine and in sleep research. The term "Actigraphy" refers to methods using miniaturized computerized wristwatch-like devices generally placed on the wrist (although they can also placed on the ankle or trunk) to record movement. Collected data are downloaded to a computer for display and analysis of activity/inactivity rhythm parameters that in turn can be further analyzed to estimate wake/sleep parameters (such as total sleep time, percent of time spent asleep, total wake time, percent of time spent awake & number of awakenings). Actigraphy provides a useful, cost-effective and portable method for assessing specific sleep disorders. The advantage of wrist-mounted actigraphy over traditional polysomnography (PSG) is convenient recording continuously for one day, several days, weeks or even longer. The disadvantage of wrist-mounted actigraphy is that the uni-dimentional signal does not identify individual sleep states. Wrist-mounted actigraphy also been shown effective in documenting the effects of various behavioral and medical interventions on sleep-wake patterns. Wrist-mounted actigraphy has limitations when applied in clinical practice, however: it is not sufficient for all clinical groups and for diagnosis of sleep disorders in patients with high motility during sleep or who have long motionless periods of wakefulness during sleep.