Actually, what is probably happening is that when you sleep longer than usual, you wake up during a different part of the sleep cycle, which can make you feel groggy. In fact, people who awaken during REM sleep, sometimes experience the REM paralysis (which kept our ancestors from falling out of trees while dreaming) as a terrible paralyzing pressure, and deaths have been known to occur in cultures which interpret the feeling as the work of demons or witches (See “The Spirit Catches You And Then You Fall Down”, about Hmong immigrants in Fresno.)