The cause of sleep is an active inhibitory process .An earlier theory of sleep was that the excitatory areas of the upper brain stem,the reticular activating system, simply fatigued during the waking day and became inactive as a result. This was called the passive theory of sleep. An important experiment changed this view to the current belief that sleep is caused by an active inhibitory process: it was discovered that transecting the brain stem at the level of the midpons creates a brain whose cortex never goes to sleep.In other words, there seems to be some center located below the midpontile level of the brain stem that is required to cause sleep by inhibiting other parts of the