I do wakeup before my alarm in the morning. I have my alarm set at 7:30 and I wake up at 7:29.<p>So I'm sure it's not about signals getting sent. It's about conditioning.
This is a common explanation to the "deja vu" moments we sometimes experience.<p>Although this does not at all explain the times where I wake up a few seconds/minutes before my alarm goes off and proceed to disable it before it does :)
Not a single data point in this theory. The reason you sometimes wake up just before the alarm (at least I wake up before my alarm because I actually check the time) is because of conditioning. Our biological clock is pretty accurate. An example of that, in university I could easily sleep on the week ends till noon. Now that i've been working for many years and waking up regularly at the same time during the week, I wake up naturally around the same time during the week end (unless I went out late the day before of course).