Boyfriend falling asleep on the phone?

So my boyfriend and I have been dating long distance for around a year and a half now, and whenever we talk on the phone late at night he always ends up falling asleep. I honestly don't know how he does it (I could never, no matter how tired I was). I'll ask him if he's tired/wants to go to bed and he'll say no, I'll tell him I love him and that I'm going to bed and he'll say no, even though he's pretty much asleep (I can tell at this point- he'll get progressively quieter and eventually just fall asleep). He told me he often closes his eyes while we're talking at night and falls asleep. I was just wondering, how is this even possible? Should I be flattered or could it potentially be a bad thing?