Your internal 24-hour sleep-wake cycle; also known as your biological clock is regulated by the brain as it responds to the amount of time spent awake and the transition between daylight and darkness.
At night, your body responds to the loss of daylight by producing melatonin, a hormone that makes you sleepy. During the day, sunlight triggers the brain to inhibit melatonin production so you feel awake and alert.
Your internal clock can be disrupted by factors such as nightshift work, traveling across time zones, or irregular sleeping patterns—leaving you feeling groggy, disoriented, and sleepy at inconvenient times. The production of melatonin can also be thrown off when you’re deprived of sunlight during the day or exposed to too much artificial light at night—especially the light from electronic devices, including TVs, computers, tables, and mobile phones.
http://www.helpguide.org/articles/sleep/how-much-sleep-do-you-need.htm#
