Mammalian sleep has evolved under the influence of the day-night cycle and in response to reproductive needs, food seeking, and predator avoidance, resulting in circadian (predictive) and homeostatic (reactive) regulation. A molecular clock characterized by transcription/translation feedback loops mediates circadian regulation of sleep. Misalignment with the rhythm of the sun results in circadian disorders and jet lag. The molecular basis of homeostatic sleep regulation is mostly unknown. A network of mutually inhibitory brain nuclei regulates sleep states and sleep-wake transitions. Abnormalities in these networks create sleep disorders, including rapid eye movement sleep behavior disorder, sleep walking, and narcolepsy. Physiological changes associated with sleep can be imbalanced, resulting in excess movements such as periodic leg movements during sleep or abnormal breathing in obstructive sleep apneas. As every organ in the body is affected by sleep directly or indirectly, sleep and sleep-associated disorders are frequent and only now starting to be understood.