Sleep is essential for health and cognitive function. It involves NREM and REM sleep stages measured using polysomnography. Common sleep disorders include insomnia, hypersomnolence, and narcolepsy. Insomnia is difficulty initiating or maintaining sleep and is treated with sleep hygiene, relaxation, and medication. Hypersomnolence involves excessive daytime sleepiness and is treated with stimulants. Narcolepsy involves REM sleep intrusion and is diagnosed by decreased REM latency on polysomnography.