Sleep Disorders
Possible causes of excessive daytime drowsiness. See Wall Street Journal article "Scientists try to Unravel the Riddle of Too Much Sleep" on December 11, 2012.
Chronic sleep deprivation
Medical and mental illnesses
hormone and vitamin deficiencies
drug and alcohol use
medication side effects
etc.
Sleep disorders
obstructive sleep apnea
restless leg syndrome
sleep-phase problems
Narcolepsy
Kleine-Levin syndrome
idiopathic hypersomnia