system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it so hard to fall asleep at night after being awake for hours but so easy to fall back asleep in the morning after just waking up and getting some hours of sleep?
gpt
One word: Melatonin.  It's a hormone released by your body which regulates your sleep cycle.  Your body produces it when it gets dark but it takes a while to build up in your system.  Then it slows/stops the release once it's light out.