system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does our perception of time disappear when we go to sleep?
gpt
Our brains stop taking in external information when we sleep. Sleep also messes with memory, so we simply don't remember much of what happened. How much we take in and remember depends on the stage of sleep. If you're sleeping well and hitting REM/deep sleep, you won't remember much. If the quality of your sleep is poor and you spend much of the night in only early stages of sleep, you'll remember more and likely be quite aware of the passage of time.

The brain is still keep track of time in the sense that it is following its circadian rhythm.