system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: How is it that someone can be exhausted yet still unable to fall asleep?
gpt
There is a health phenomenon called being "overtired", which is caused by insufficient or restless sleep, lack of a proper sleep rhythm and staying up for longer than your rhythm would dictate.

It causes this kind of insomnia through many unexpected changes in your brain chemistry when you don't get the sleep your body expects. Specifically, a hindered production of melatonin.