Being excessively sleepy during the daytime can be caused by not getting enough sleep at night even though you may lay in bed all night and think you're getting sleep you still may be waking up at night and not actually knowing about it.
Also some people may actually need more than 8 hours of sleep so maybe try sleeping in a few hours longer to see if the extra sleep helps you feel less tired during the day.
If not you may have a sleep disorder which you should see your doctor about.