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you preferably need 8-9 hours perhaps.
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it could also be sleep quality. Are you stressed? is the quality of your sleep good?
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fainting? Have you seen a doctor/gp. contact one perhaps.
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maybe some things to make you sleep better. turn off the lights, have your room dark, and don't use electronic devices your blue light can affect you. don't have caffeine, alcohol, or heavy meals before. Have a comfy bed. Exercise before like in the evening could also help.
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drink caffeine when your tired, will make you more alert as I assume you are in yr 13. so the caffeine will be brief. melatonin could work. I'm not sure when you fall asleep, but if it's delayed then yeah.