i assume you've looked at the food you eat, are you eating the right things? what time are you actually going to bed in the evening? Are you feeling under pressured with anything ie course work, girlfriends etc?
Apart from saying to you, make sure you have 3 decent meals a day (including fruit, veg, orange juice), take a 10minute walk every day, stay off coffee + tea coz in the long run it makes you more tired once the caffine's worn off, and be in bed by no later than 12, make sure you have a minimum sleep of 7-8hours per night - i dont really know what to advise personally, sorry