system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes people to wake in the middle of the night, without any disturbance?
gpt
We have natural sleep cycles that are generally around 3.5-4 hours.  At the end of the cycle, you're susceptible to being awoken by any number of things.  Most commonly I've found it to be dehydration, having to pee, too cold, too hot, or a loud noise from a pet/person/thing indoors or outside.  Trick is, if it's just a loud car driving by or someone cursing at their own mortality, it usually takes you longer to become fully conscious than it does for the noise to cease, leaving you wondering why you woke up.