I use alarms out of convenience and convention.
But the weird thing is, I have been able to set my own internal alarm for decades, now. So long as I know the current time, I can focus on the idea of waking up at a specific time, visualizing the act of waking up and getting clear out of bed right as my clock rolls over into that exact time, and actually wake up within a 5-minute window around that time.
Only strong sleep deprivation - less than 3-4 hrs of sleep, or less deprivation but chronically so over the previous few days - can impact this ability. Or to be more honest: I still wake up, on time, but then usually just fall right back to sleep due to a lack of inertia.
But then again, I also have the ability to just wake up, sleep deprivation or fully refreshed, and just pop out of bed like a piece of toast. As in, I can go to full wakefulness and full functionality within seconds so long as I can catch myself and prevent myself from falling asleep again.
