Wiktionary
drift off
vb. 1 (context intransitive idiomatic English) To fall asleep slowly. 2 (context intransitive idiomatic English) To lose concentration.
WordNet
drift off
v. change from a waking to a sleeping state; "he always falls asleep during lectures" [syn: fall asleep, dope off, flake out, nod off, drop off, doze off, drowse off] [ant: wake up]
Usage examples of "drift off".
He had followed along behind the Reed twins as they headed through the woods toward the path, and had allowed his mind to drift off because the boys were moving with agonized slowness, trying not to rustle a single bush or snap a single twig.