8 Hours is good enough amount of sleep for a healthy teenager / adult. Maybe because of strain on eyes (due to studies, or computer use) you may feel fatigued. Also it could be because of diet. You can use the following tips to be fit and feel more alert.
- Improve your diet. Do not eat oily food, or late night snacks. Eat meals at proper times.
- Go for short walks during any suitable time of the day.
- Do not unnecessarily strain your eyes by watching TV, or using computer too much.
- Take short breaks between studies.