Thanks to everyone who responded -- For reference, that 42 was while awake, though quite relaxed. In fact, I'm sat at my desk typing this and my pulse is only 51 now.
I think, since I'm not overly fatigued/worried, I'll just tack it onto the list of things to ask GP next time.
I'm not the most athletic of people, yet this is not an uncommon sight on my garmin app, and I have confirmed it by manual measurement.
Dear lazyweb, should I be asking my doctor about bradycardia?
Just in case, here're some more details -- The `#sequoia` IRC channel is on the Freenode IRC network (See freenode.net/kb/answer/chat if you don't know how to access that) and you'll want to speak with `neal` I imagine. I'm also on there as `Kinnison`.