Opponent process model

Theory of two independent, and opposing, regulators of sleep and wakefulness in the human body.  The biological clock is responsible for, among many other things, keeping us awake.   The sleep drive promotes sleep.   When clock-dependent alerting is very high, such as in the mid-morning and the early evening, it is very difficult to fall asleep, unless there is significant sleep debt.  However, when the clock's alerting slacks off, which it does typically in the mid-afternoon, the sleep drive will take over, especially if there is a large sleep debt.  The opponent process model explains not only why the mid-afternoon siesta can be so hard to resist, but also underlies the phenonomen of jet lag. 

Source:

Dement, The Promise of Sleep