 It follows that the evolution of monophasic sleep may hitherto be an unknown advantage of evolving larger mammalian body sizes and therefore lower BMR.Sleep is sometimes thought to help conserve energy, though this theory is not fully adequate as it only decreases metabolism by about 5–10%.Additionally it is observed that mammals require sleep even during the hypometabolic state of hibernation, in which circumstance it is actually a net loss of energy as the animal returns from hypothermia to euthermia in order to sleep.