Research data suggests that adenosine is indeed an important endogenous, homeostatic sleep factor and it is likely to be mediating the sleepiness that follows prolonged wakefulness because it acts as a direct negative feed-back inhibitor of neuronal activity. Adenosine is proposed to act as a homeostatic regulator of sleep and to be a link between the humoral and neural mechanisms of sleep-wake regulation. Both the adenosine A(1) receptor (A(1)R) and A(2A)R are involved in sleep induction.