Animals, particularly warm-blooded animals like mammals and birds, use a lot of energy to maintain body temperature and other bodily functions, it is argued, and the waking brain in particular uses a great deal of energy, which needs to be recuperated. During sleep, our core temperature is reduced by around 1°C, and so we use less energy maintaining our body temperature, suggesting that one of the primary functions of sleep may be to conserve energy. The energy conservation theory is also supported by the fact that, in general, metabolic rates are higher in smaller animals, which typically sleep longer hours, and that cold-blooded animals like reptiles and fish tend to exhibit less obvious and unequivocal sleep.
However, it is actually only during non-REM sleep that the brain uses less energy at all, and brain wave activity during REM sleep is very similar to (or sometimes even higher than) that of the waking state. In fact, sleeping only reduces metabolism and energy use in humans by at most 5-10% overall - by some estimates, 8 hours of sleep saves about the amount of energy in a slice of toast - suggesting that simple inactivity or resting while awake would serve this purpose just as well, although others have argued that even a small energy saving can have an important impact on evolutionary selection and survival. Hibernation, on the other hand, is a truly energy-saving state, although it is a very different phenomenon to sleep.
In addition to this, sleeping puts us in a very vulnerable and potentially dangerous state of unresponsiveness, which, in evolutionary terms, seems like a high price to pay for such small energy savings. All this suggests that energy conservation is perhaps not the main purpose of sleep, as was once believed, although it may be at least a secondary one.