The neural relationship between light and sleep
(PHYSorg.com Medicine & Health)

Humans are diurnal animals, meaning that we usually sleep at night and are awake during the day, due at least in part to light or the lack thereof. Light is known to affect sleep indirectly by entraining-modifying the length of-our circadian rhythms and also rapidly and directly due to a phenomenon known as masking. But while a great deal is known about how light affects circadian rhythms, little is known about the direct effects of light on sleep: Why do we tend to wake up if the lights are flipped on in the middle of the night? Why does darkness make us sleepy? Caltech researchers in the laboratory of Professor of Biology David Prober say they have discovered at least part of the answer: a specific protein in the brain that responds to light and darkness to set the correct balance between sleep and wakefulness.