Dust in the smoke alarm perhaps ?

Ooh, I do hope so. The last thing I did before going back to bed the second time was to blow into the thing, so if that's the answer then hopefully it won't recur for a while.

A colleague suggests that smoke alarms get progressively more sensitive as they get older (since they work by means of smoke drifting between a radiation source and a sensor, so as the source decays it takes less and less smoke to bring the sensor results down far enough to trip the alarm). If you're right and he's wrong, it might mean I don't need a new alarm :-)