system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Sometimes when we sleep, we wake up feeling very refreshed and energized. Sometimes we wake up feeling very sore and stiff. Assuming that the same amount of sleep was needed in both instances, why do we sometimes wake up sore and groggy?
gpt
you actually "deep" sleep in ~1.5 hour cycles. if you wake up in the middle of one of those cycles you'll feel groggy. if you time it to wake at the end of those cycles, you'll feel more refreshed.

in your scenario where you've slept the same amount of time, i'd guess something was disturbing your deep sleep, but not enough to "fully" wake yourself up.

there are some alarm clocks/apps that monitor your sleep (via movement) and will wake you up when you are not in a deep sleep cycle.