Many theories of sleep have been advanced to explain the temporary loss of consciousness which we know as sleep. The oldest theory is that sleep is induced by a reduction in the blood supply to the brain or at least to conscious centres. This is known as ischemic theory.

Even the ancient Greek physicians were aware that the carotid artery was in a way concerned with the onset of sleep. The name itself expresses this belief. The Greek word 'Karotides' for carotid arteries is derived from karoo which means