Taken from the Power of Sleep:  Recent research indicates that the human body is inclined to rest in the middle of the afternoon as well as at night, even after adequate nocturnal sleep.  A heavy meal at lunch doesn’t make you sleepy, it simply unmasks the physiological sleepiness that’s already in your body.  The “postlunch dip” in alertness occurs whether or not food is consumed.  A tendency to get sleepy at certain times during the day is biological.  The afternoon trough occurs approximately 12 hours after the middle of your night’s rest.  It’s then that you need a nap the most, especially if you have slept poorly the night before.  And, remember, a nap should only be 15-30 minutes in duration.  Sweet Dreams!