Normal sleeping is the body mechanism to rest and rejuvenate for the next day. Our brains need time to sort our all the information and clatter it received during the day. During sleep it is able to do that. That is we when we wake up in the morning, we feel fresh and rested.
Abnormal sleeping,where one sleeps excessively, could be because of an illness e.g. Sleeping sickness caused by tse-tse fly, the sickness affects the central nervous system and makes one lethargic. Also when people are sick, they generally feel tired quickly and sleep more often.