From the site http://www.brainrules.net/sleep
Napping is normal. Ever feel tired in the afternoon? That’s because your brain really wants to take a nap. There's a battle raging in your head between two armies. Each army is made of legions of brain cells and biochemicals –- one desperately trying to keep you awake, the other desperately trying to force you to sleep. Around 3 p.m., 12 hours after the midpoint of your sleep, all your brain wants to do is nap.