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Sleep is a periodic condition of the body and especially of the body and especially of the nervous system. There is a natural rotation of sleeping and waking every twenty-four hours.Many theories have been advanced as to the caused sleep.One theory depends upon the well-known bad that the brain receives much smaller blood supply during sleeping than in the wakingstate. Another theory is the chemical one stating that sleep is due to want of oxygen in the nerve centers. A third theory raises the mind remouns active during the period of sleep. When sleep comes on the eyes are closed the sense of sight is quickly lost as the sleep deepens. The pupils contract also during sleep and dilate widely as person wakens. Hearing is lost more showly and a person can be wakened even from deep sleep by a loud noise. In natural sleep touch remains the least affected of senses. Memory and imagination temain longest and in dreams the mind is presented with a series of bright, unconnected pictures.