[kempbellt]

An alarm going off if a very disruptive disturbance to your sleep cycle, and it is predictable.

Your subconscious mind perceives it as a predictable negative event and wakes you up in an attempt to prevent the disturbance.

Goes to show how cool our brains are.

[thedanbob]

After I got an Apple Watch I started wearing it to bed and using it as my alarm. I can't remember ever waking up just before it goes off although that would happen a lot with my old alarm. Perhaps because a vibration on my wrist, while still effective at waking me up, is not nearly as disruptive and negative as an alarm.

[kempbellt]

>is not nearly as disruptive and negative as an alarm

My personal experience says this is likely the cause as well. A vibration on your arm is a lot less disruptive as "EENENENNNG..... ENENENENNG....ENENNENN" right in the middle of a dream.