system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come if you stay up late because you don't feel tired you will still feel exhausted the next morning?
gpt
A few reasons. Easy one is that just because you didn't feel tired doesn't mean you aren't tired. Our bodies are sometimes shit at telling our brains what they need. 

Another possibility that comes to mind is based on sleep cycles. You typically go through two sleep cycles per night of about 3-4 hours in length. If you wake up at the end of a cycle you feel great. If you wake up in the middle of a cycle you feel shit. So, on a typical day you get say 7 hours sleep, wake up after two 3 1/2 hour cycles and feel great. Then you stay up a couple hours and only get 5 hours sleep. Now you've had 1 1/2 cycles, woke up in the middle and feel shit.