Sleep cycle issue. To reset your sleep clock, go to bed at the same time every night, awaken at the same time every morning, don't nap during the day, use the bed for sleeping (not eating, TV watching, reading, etc), if you awaken during the night, stay in bed until your determined awakening time. Sounds like your sleep clock is off for some reason.
Sleep hygiene. Ask yourself some simple questions. Is sleep trouble new or chronic? Is room dark, quiet ; cool? Is there any new stress? Any change in activity level? Any new medications? How much caffeine do you consume? Right before bedtime? How much fluids do you drink (right before bedtime)? Do you snore? Wake up w/headache? Fall asleep inappropriately during daytime? Finally don't exercise right b4 sleep.