hey im a 18 yr old guy...happens to me occasionally too...my parents always told me im growin and my hormones are acting up...seems to fit because i always grow a bit following the exhaustion periods and i find my body has matured in some way as well....if you wanna stay awake keep your body moving and dont lay down or get into a comfortable relaxed position, keep active and just get to bed a lil earlier if you can
I have somewhat of the same problem you do. I am an 18 year old female.
I can go to bed at like 1230, set my alarm for about 830 or so...and sometimes I sleep right through my alarm, or I will wake up, and turn it off and go back to sleep, cuz I feel like I havent gotten enough sleep.
But then other nights, I can go to bed at like 530am and wake up 3 hours later, and im good to go for the rest of the day! I hate it! I dont understand it.