Extremely tired but cannot sleep?

i usually sleep 8 hours a day. But last night i only slept 4 hours because I had to study. As a result, after that night, I was tired for the whole day so i i decided to sleep early... but only was able to sleep for 2.5 hours. I woke up for a few hours then i felt extremely tired with headache. I decided to go back to sleep but I've been trying to sleep for 3 hours now with no luck. How can i fall asleep?
Extremely tired but cannot sleep?
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