Sleep is defined as a state of reduced motor activity (movement), lower response to sensory stimulation, adoption of postures such as lying down with eyes closed and easy reversibility (when compared to coma or hibernation). Sleep is essential for the normal functioning of all systems in the body. Sleep has direct effect on the individual's mental and physical health. Lack or poor sleep can affect the individual's ability to fight and endure sickness, cause irritability, poor concentration, impaired memory and decreases hand-eye co-ordination.

In the past, sleep was considered a passive state (where nothing happened), however it is now known as a dynamic process (brain activity continues during sleep). In humans cells produce and release proteins essential for growth (especially in children) and tissue repair during sleep. Sleep helps humans maintain optimal emotional and social functioning while awake by giving a rest during sleep to the parts of the brain that control emotions and social interactions.