This is a very interesting questions, many doctors have been asking themselves for ages: What exactly controls a sleep/awake cycle in our brain? There are a number of studies have been conducted to understand better that mechanism.
The main purpose of these studies is to understand better how sleep disorders start and how they are progressing, and also what the reason is for this.
These studies have helped to establish, that there is a couple of mechanisms which affect this cycle ( sleep/awake). The first one is sleep cycle, the second – wakefulness cycle.
Provigil, the medicine, very commonly prescribed for the treatment of sleep disorders, increases wakefulness in patients.