Here are some possible reasons for feeling sleepy or tired:

 - Not getting enough sleep - When you are young, you need 8-10 hours per night to feel rested. Try going to bed 
earlier and avoid screens before bed.
 - Poor diet - Eating too much junk food and not enough fruits, veggies and protein can leave you low on energy. Small changes like more veggies, fruits and drinking water can help. Also, avoid eating close to bedtime.
 - Stress - School , relationships, family issues  etc can drain you. Try to find healthy ways to manage stress like exercise, talking to friends or meditating.
 - Medical issues - Things like anaemia , thyroid problems or even depression can cause fatigue. If lifestyle changes don't help, it's best to see a doctor or clinic to get checked out.
 - Not enough physical activity - Exercise gives you energy! Aim for 30-60 minutes of activity per day. Even a brisk walk can perk you up.
 I know feeling constantly tired is frustrating. Try these tips for a few weeks to see if your energy improves. If not, do not hesitate to see a healthcare 
provider. They can help identify any underlying medical issues. Take care!