When scientists began using electroencephalograms (EEGs) to monitor the brain`s electrical patterns during sleep, it became clear that sleep is an active, dynamic state. Many of the body`s major restorative activities, including tissue repair, muscle growth, protein production, and growth hormone release happen mostly, or only, during sleep. The brain may need sleep to enable it to function well during wakeful hours. Animals that are completely deprived of sleep lose all immune function and die in a matter of weeks. In humans, too little sleep leaves us drowsy and unable to concentrate the next day and also impairs our physical performance.