A Gizmodo news report discusses the impact of light on sleep and wake patterns.
The physiological processes that control the daily cycle of sleep and wake, hunger, activity levels, body temperature, melatonin level in the blood, and many other physiological traits are called the endogenous circadian rhythm.
On its own, the endogenous circadian rhythm is nearly, but not exactly, 24 hours. Our bodies rely on the Sun to reset this cycle and keep it at precisely 24 hours, the length of our days. The light – and the dark – are important signals for the cycle. This circadian rhythm has developed over three billion years as life evolved on Earth in the context of the Sun’s day/night cycle. It is built deeply into our genetic makeup.