This is particularly true since one aspect of sleep is decreased responsiveness to the environment.
The role that sleep plays in evolution is still under study. ( ① )
One possibility is that it is an advantageous adaptive state of decreased metabolism for an animal when there are no more pressing activities. ( ② )
This seems true for deeper states of inactivity such as hibernation during the winter when there are few food supplies, and a high metabolic cost to maintaining adequate temperature. ( ③ )
It may be true in daily situations as well, for instance for a prey species to avoid predators after dark. ( ④ )
On the other hand, the apparent universality of sleep, and the observation that mammals such as cetaceans have developed such highly complex mechanisms to preserve sleep on at least one side of the brain at a time, suggests that sleep additionally provides some vital service(s)
for the organism. ( ⑤ )
If sleep is universal even when this potential price must be paid, the implication may be that it has important functions that cannot be obtained just by quiet, wakeful resting. * metabolism: 신진대사 ** mammal: 포유동물