I am a baker and have to be at work at 5 am, and I would be late every single day if I didn't use my alarm. How did bakers or other early riser tradesmen (and women) make sure they were up on time?
This is a pretty common question here on AskHistorians, it's part of the subreddit's FAQ
/u/mimicofmodes has also answered this question before.
/u/frogbrooks has previously explained how to wake up in the predawn during Ramadan before alarm clocks. (this answer is in the FAQ but especially interesting).
EDIT: Even though it's not medieval, you might like what /u/toldinstone has to say about Ancient Romans waking up without alarm clocks.