We know that you can kill rats with severe sleep deprivation and there have been extremely rare cases of fatal insomnia in humans. Rest from activity on a daily basis is a fairly universal trait among all animals. The cold-blooded creatures rest but the brain structures to support sleep are largely absent. There is no convincing evidence of sleep with its unique brain activity amongst reptiles and lesser animals. The body organizes itself to accomplish specific tasks at set intervals within the 24-hour day.
Darkness is the preferred time for sleep and rest.
Our body prepares for sleep by orchestrating a complex set of physiological changes that favor wake activity in daytime and sleep at night in a rhythmic manner. The net results is physiological support of the sleep process as our daytime activity ends. Sleep induction is also favored as our sleep debt reaches a significant level after about 16 hours of wakefulness. The merging of our sleep debt pressure with physiological support of sleep results in a strong sleep urge at bedtime. Sleep is associated with quiescent behavior, a reduced responsiveness to stimulation, profound changes in brain electrical patterns and significant changes in physiology.