well, maybe your room's too cold, lower the cooling level of your A/C. Sometimes, due to very chilled atmosphere, you may not feel comfortable while sleeping.

alternately, try sleeping a little late...often it happens that sleeping early, even when not tired or exhausted, and having longer hours of sleep will also make one feel tired...I experienced that a couple weeks ago, slept on the weekends for 8 hrs, and then felt way too tired during day time..

hope mi suggestions help :)