Why do I feel tired around 8 pm if I never stay up?

the body adjusts to your habits, so if you commonly sleep before 8pm your body will yearn for sleep after 8pm. if you sleep later your body will adjust to new time in a week or two

Tsee fly bite, poor nutrition, lack of exercise, to much sleep to name a few.... to little sleep, not enough sunshine.... to much tv.... all can make you feel tired.