system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how does your body know when to wake up from sleep?
gpt
Hormones are found at different levels in your blood at different  times of the day, when melatonin peaks it makes you wanna sleep, melatonin levels eventually go down and cortisol (also known as the stress hormone) goes up, making you wake up at the end of your sleep cycle. You can see how alarm clocks often disrupt the natural sleep rhythm by interupting the sleep cycle when it hasn't ended yet, causing you to feel groggy/sleepy when you wake up.