It could have been an internal clock doing the work there; I can usually wake myself up more or less when I want by just thinking about it before I go to sleep, so there must be some kind of internal timekeeper running as a background task.

(n.b.: I can do this, but it’s not like I trust it for important wake-up times; I set an actual external alarm for those. Maybe it would work anyway, but I’m not taking any chances)