Recent theories indicate that deep sleep may help decrease synaptic strength which increases during the day as an organism learns from interacting with the environment. Sleep may then preserve memories while preparing the brain to function well another day. REM sleep, which occurs predominantly in the very early morning hours, may function to allow the brain to become metabolically as awake as possible while yet remaining asleep, in order to facilitate waking up in the morning. (Just think about how different you feel when awakened in the middle of the night from deep sleep as compared to waking from a dream in the morning.) If sleep helps conserve and restore energy and helps refresh the brain, the question still remains- why did the strategy of spending long periods of time sleeping evolve? A brain could sleep for short periods of time rather than shutting down for a longer time. The answer is likely that given the vulnerability of being awake all day, animals are actually safer if they sleep all at once, rather than if they break up sleep over a 24 hour period. Of course, animals and humans are able to sleep briefly and maintain wakefulness over long periods of time when necessary, but this does not lead to optimal functioning. It does; however, seem to be an effective adaptation in times of severe stress. Interestingly, birds have developed strategies in which one hemisphere of the brain may sleep while the other remains alert. Whales and seals also show uni-hemispheric sleep. Because they have a different evolutionary lineage from birds, it seems that they thus evolved this ability separately. Uni-hemispheric sleep may have developed as a way of dealing with the danger of predators that can suddenly approach from any direction in the air or in the sea. In humans and mice, decreasing deep sleep may be a coping mechanism for dealing with the danger and stress of having predators nearby. This certainly fits with the experience of insomnia, during which people have lighter sleep due to stress and over-arousal.