Sleep is a gut response
While it is common knowledge that physiological reasons - a good meal, an inherent biological clock, or fatigue - induce sleep, how did sleep evolve in the first place? An interesting theory on the evolutionary origin of sleep, that relates this nodding habit to gut immunity, has been published recently in Medical Hypothesis (1995).
According to this theory, sleep evolved in mammals mainly to protect the brain from damage caused by continuous wakefulness. Alert brain function demands a good amount of blood supply, which is taken care of by an increase in permeability of the blood-brain barrier (the membranes that separate the blood vessels from brain cells) during wakefulness. But continuous increase of this permeability can lead to damage of brain cells or nervous tissues. The blood-brain barrier may also come under stress from the omnipresent gut-associated bacteria; the cell walls of these bacteria have constituents that induce cellular responses in the human host, which also increase the permeability of the barrier.
"In order to prevent this from happening, an immune-like response specific for the brain has co-evolved along with the symbiosis between animals (including humans) and their gut bacteria," says C Korth, a British scientist and the proponent of this theory. In other words, sleep could have evolved to temporarily bring back the permeability of the blood-brain barrier to normal levels, so that it can increase again during alertness.