In humans, sleep plays a key role in vital restorative functions such as muscle growth, tissue repair, protein synthesis, and growth hormones, which are released almost entirely during sleep.
When you are awake, neurons in your brain produce adenosine (a metabolic byproduct of cellular activity, this molecule not only controls the flow of information through neural circuits in the nervous system but also acts as a homeostatic regulator in the brain).
An excess of adenosine is associated with an individual’s perception of being tired and to the drive to sleep. As long as you are awake, adenosine accumulates in your brain. However, during sleep, the body has the ability to clear the adenosine from your system and, as a result, you feel alert and rested upon awakening.