system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How does waking up work?

What makes our body decide, ok now it's time to get up? Why do things like nightmares often allow us to shortcut sleep and wake up early?
gpt
Our circadian rhythm dictates our sleep/wake cycle. And it's primarily driven by light and temperature. Once the sun goes down, our bodies start releasing melatonin, preparing us for sleep hours before we're actually ready to go down. Cooler night temperatures also aid in this.

As the sun rises and temperatures increase, our bodies stop producing melatonin, allowing our bodies to wake naturally and slowly over time. Even with your eyes close, you're still receiving light through your eyelids.

It's another reason people say if you want to fix your circadian cycle you should go camping for a few days and get away from all the artificial light in your life.

Your body is still able to receive external input while asleep, to protect you from danger. Hence why certain noises might wake you in the middle of the night. Adrenalin helps get your body going in stressful situations while awake and also asleep. However, at night, you've still got melatonin coursing through you, so it's relatively easy to go back to sleep. If something startles you awake after the sun is up, your body has already slowed or stopped the production of melatonin and it's harder to get back to sleep or stay asleep.