A sleep disorder is any medical condition that affects the quality, timing, duration, or architecture of sleep, and which causes distress or impairment in daily functioning. Sleep disorders are common, affecting a significant proportion of the global population, and they can have far-reaching consequences for physical health, mental wellbeing, and quality of life. Sleep disorders are broadly categorized into several groups. Insomnias involve difficulty falling asleep, staying asleep, or waking too early. Sleep-related breathing disorders include sleep apnea and snoring. Hypersomnias of central origin, such as narcolepsy and idiopathic hypersomnia, are characterized by excessive daytime sleepiness. Circadian rhythm sleep-wake disorders occur when the internal clock is misaligned with the desired sleep timing, as with jet lag or shift work disorder. Parasomnias are abnormal behaviors or experiences during sleep, including sleepwalking, sleep talking, sleep paralysis, nightmares, and REM sleep behavior disorder. Sleep-related movement disorders include restless legs syndrome and periodic limb movement disorder. Sleep disorders can be primary — arising independently — or secondary, resulting from another medical, psychiatric, or neurological condition, or from medication use. Common contributing factors include stress, anxiety, depression, chronic pain, hormonal changes, and lifestyle factors. The consequences of untreated sleep disorders can be serious, including increased risk of cardiovascular disease, metabolic disorders, impaired immune function, accidents, reduced cognitive performance, and mental health problems. Diagnosis of sleep disorders typically involves clinical evaluation, sleep diaries, questionnaires, and in many cases a sleep study (polysomnography or home sleep testing). Treatment depends on the specific disorder and may include behavioral therapies, medication, lifestyle changes, or medical devices.