How do you know when to sleep?
Your waking/sleeping cycle is usually 24 hours long. If you were kept from knowing the time of day, this cycle would gradually lengthen to about 25 hours. So you must have your own internal body clock. Your body clock is related to an area of your brain called the hypothalamus, part of which is directly connected to your eyes. Scientists have found genes that affect the body clock: animals with faulty 'clock' genes have different sleep/waking cycles.