A more anthropological and adaptive theory holds that sleep improves an animal’s likelihood of survival, and particularly, that those animals with sleeping habits appropriate to their environment are most likely to survive. It is based on the idea that a sleep period ensures that animals are safely and quietly hidden away at the very time of the day when they would otherwise be most at risk from predators.
However, although this theory might explain a period of quiescence or inactivity during times of danger, it does not explain why sleep should leave us so vulnerable and defenceless (with greatly decreased sensitivity to external stimuli, and sometimes complete paralysis) at such a critical time. Intuitively, it would seem that remaining conscious would make us better able to deal with any threats or emergencies. Neither does it explain why carnivores like lions, which have few predators to fear, sleep such long hours.
Also, we now know that sleep is not just a passive removal from the environment. Indeed, it appears to be an actual drive in most species (animals will alter their behaviours in order to make sure they obtain sufficient sleep) and the preservation and protection theory fails to deal with this kind of behaviour. It is possible that, while such a theory may have been valid long in the evolutionary past, sleep in higher animals has evolved over time to such an extent that it no longer fulfills the same functions.