The most obvious reason is that you're not sleeping well enough or long enough. Make sure you're going to bed early enough that you can stay in bed at least 9 hours. This should help make sure you're getting at least 8 hours of full sleep time, plus a little wiggle room to account for time to fall asleep and some time if you wake up in the middle of the night. If you continue to feel sleep even after 7-9 hours of sleep, then consult your healthcare provider to rule out other reasons why you're feeling sleepy.