Get a cheap old fashioned alarm clock (we’re talking about something that costs maybe 10 bucks).
Put it out of reach so that you have to physically get out of bed to turn it off.
Configure it to go off at the appropriate time with the nastiest sound (usually they have an “alarm with a radio” and an “alarm with alarm sound” modes and you definitely want to have it in the latter mode, not the former).
It’s a pretty horrible way to wake up if you went to bed late (protip: stop drinking coffee and using a computer after 11PM to deal with the whole only falling asleep late part of the problem) and that’s why it works.