Dumeril's Diurnal Cycle
I rarely use an alarm clock.
I'm one of those blessed/cursed people who always wake up at the same time everyday. It doesn't really matter what time I go to sleep, somehow my body knows when to wake up. It takes me a few days of waking up at a particular time and my body is then trained to wake up at that time from then on (assuming I've gotten sufficient sleep). Right now, I'm programmed to wake up at 6AM.
It's a blessing on weekdays because I can get going for work and not disturb my wife. It's a curse on weekends when I'd like to sleep in but I can't. On vacation, I often find myself having to go for walks in the morning while my family sleeps in the hotel room because the only other option is to stare at the ceiling in the dark while listening to everybody else sleeping.
The two exceptions to this are when I travel across time zones and when daylight savings time begins or ends. In both cases, my internal clock is out-of-sync with the new time and I have to have a re-training period. But with the COVID moratorium on travel and now that my body has acclimated to the recent DST event, you could set a clock by my circadian rhythm.