Sleep can be measured in several ways. Polysomnography, or a sleep study, is the gold standard: an overnight test that records brain activity, breathing, heart rhythms, and movement. Home sleep testing offers a simpler alternative focused on breathing but may miss some conditions. Wrist-worn actigraphy estimates sleep from movement, while sleep diaries provide a daily log of subjective sleep patterns. Common metrics include sleep efficiency, the percentage of time in bed actually spent asleep, ideally 85 percent or higher; sleep onset latency, the time it takes to fall asleep, normally 10 to 20 minutes; and WASO, or wake time after sleep onset. The Epworth Sleepiness Scale is a questionnaire used to estimate daytime sleepiness, while microsleeps are brief involuntary episodes of sleep that occur when someone is sleep deprived.
Sleep disorders span a wide range. Insomnia involves difficulty falling asleep, staying asleep, or achieving restorative sleep; acute forms are short-term and situational, while chronic insomnia lasts three or more nights per week for three or more months and affects about 10 percent of adults. Obstructive sleep apnea involves repeated breathing pauses caused by airway blockage, with severity measured by the apnea-hypopnea index, where 5 to 15 events per hour is mild, 15 to 30 is moderate, and 30 or more is severe. Central sleep apnea is rarer and stems from the brain failing to signal the breathing muscles. The gold-standard treatment for obstructive sleep apnea is CPAP, which delivers pressurized air through a mask to keep the airway open, with BiPAP used for more complex cases. Common symptoms include loud snoring, gasping, daytime sleepiness, and morning headaches.
Other disorders include restless legs syndrome, which creates an irresistible urge to move the legs especially at night, and periodic limb movement disorder, characterized by repetitive limb movements during sleep. Narcolepsy is a neurological disorder of excessive daytime sleepiness with disrupted REM regulation, sometimes involving cataplexy, sudden muscle weakness triggered by strong emotions, and linked to deficiency of the wakefulness-promoting neurotransmitter orexin, also called hypocretin. REM sleep behavior disorder involves acting out dreams due to lost atonia and often precedes Parkinson's disease and related conditions. Parasomnias are unusual behaviors during sleep and include sleepwalking and sleep talking, both arising from deep NREM sleep, as well as night terrors, which are episodes of intense fear in NREM that the sleeper typically does not remember, distinguishing them from REM-stage nightmares, which are vividly remembered. Sleep paralysis is a temporary inability to move while transitioning between sleep and wakefulness, and the hypnagogic and hypnopompic states are the transitions into and out of sleep, respectively, when vivid imagery can occur.