system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do you sometimes wake up with a headache if you've slept for too long (i.e. more than 9 hours)?
gpt
It's usually dehydration. 

When you go 9 hours or more without drinking water you become dehydrated regardless of whether you're awake or asleep. Try drinking a glass of water before you go to bed and keeping another full glass by your bed so you can drink if you wake up during the night.