I currently have a natural alarm clock.
I set mine anyway, but I find that I don't need to.
See, every six hours I'm meant to take my pain meds. And they last almost exactly six hours. So I pretty much know when it's getting close to time for an other pill. It's nothing dramatic, but I start to feel a little twinge of "inside pain". This would be opposed to "outside pain" which is from swelling. That disappears as soon as I elevate my leg. "Inside pain" comes from just inside my kneecap, which is where they've drilled.
So, this morning I woke up at a strange hour. My knee was feeling a little sore, so I iced it. Then it dawned on me... hrm... did I forget to set my alarm? I took a look at my clock and Yup... 6:20am. 20 min overdue.