I think the assumption that people fall asleep during the daytime because they are bored is mostly a myth - it is more because they are especially tired or (like others said) something physiological is causing it. Falling asleep during a one-on-one conversation is pretty extreme and rare IMO, probably more likely to be caused by a physical condition rather than anything else. I definitely would not take it personally but would not see a T who falls asleep in session either, physical limits or not.