The sleep sistem results of the activity of two components: a descending component originated in the limbic structures, and an ascending system involving the bulbopontine structures which receives projections from the spinal cord components. Although the neocortex is not necessary for sleep mechanisms, it plays a very important role in sleep. Therefore, during the sleep state there is no significant quantitative difference in brain activity, but qualitative changes are recorded in EEG of epileptics patients during all night sleep.