system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it tough to sleep at night even when you're tired, but easy to fall back asleep in the morning when you've just had a good night's sleep?

It seems like it should be really easy to fall asleep after going 16 hours without sleep, but often people just lie in bed for an hour or more, and then after 8 hours of sleep, they have trouble staying awake enough to get out of bed.  How does this work?

Edit: Thanks to node_of_ranvier for the explanation!
gpt
It has to do with your heart-rate. When you're going to bed after 16hrs without sleep your heart is probably still pumping blood around pretty quick. Especially if you've had coffee, an energy drink or some other form of caffeine to keep you awake. However when you wake up after having been asleep for a solid night of rest, your heart is beating at a much lower pace and until you raise your heart-rate back up (by doing some physical activity such as standing up out of bed or masturbating) you'll find it's very easy to go back to sleep.