your sleepy because you slept for too long. i know that sounds funny, but when you sleep you have cycles of REM sleep and normal sleep, and the longer you sleep, the longer the periods of REM last. But REM sleep is about as exhausting as being awake, so if you sleep more than about 8 hours, the REM cycles get too long, it makes you feel crappy and then you want to sleep more, but the more you sleep, the worse you feel. So, keep the sleep under 8 hours and naps under an hour and you'll feel better!
