system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how are we sometimes able to wake up a second before the alarm goes off?

It’s always sudden too, “bam wake up”. One second later, beep beep beep. (Cell phone alarm in my case)
gpt
The body has an internal clock, simply stated.  It is aware of the time of day almost to the minute.  We don't have access to that clock consciously, but it's there.

You can sometimes make it wake you up if you're focused enough on waking up at that hour while you fall asleep.  This is assuming you're not exhausted enough for the body to ignore your requests.