If I don’t set an alarm I always wake up much earlier, and grab my phone in fear that it’s too late. Setting an alarm at a very reasonable tim (ie >8h after going to bed) helps me sleep calmly, knowing that I will not oversleep.
I have the opposite problem. I have to sleep with a watch. Otherwise if I wake up and can't check how much time I have left to sleep, I get anxious that the alarm is about to go off and I can't get back to sleep.
It’s interesting how different minds work. :)