Time change - didn't miss it!

I woke from a dream at 1:58 AM (EST) early on Sunday morning and looked at my clock to check the time. Naturally, I kept watching for a couple of minutes more, to see the switchover from 1:58 to 1:59 to 3:00 (EDT). The clock worked, as expected. More to the point, it was fascinating to note the fact that I woke up spontaneously at 1:58 to enable me to watch it. I'll generally wake up before an alarm when I've set one in order to meet some commitment, but I hadn't done so in this case and hadn't (at least consciously) declared myself as committed to supervise the clock's progress in, er, real time. Why and how did I wake so precisely? I accept that I must have a relatively fine-resolution body clock, but what tripped its alarm setting?