I nearly passed out at MIL's last night. I think it was the combination of an earlier-than-usual daytime nap, and a ridiculously sugary dessert.
What makes it worse is when I get the opportunity to sleep, I can't do it. If I have a daytime nap I feel I should listen to a Hypnobabies CD... and I do usually fall asleep in the middle, but it's a distraction for the first 20 minutes or so. Or I start reading a library book because I know I have to return them soon (and because I'm addicted to reading). Or I start worrying about all the things I have yet to sew/knit/clean/cook in preparation for the baby, and make a mental list of what I'll do when I wake up, and feel guilty about napping at all...
And then I wake up early in the morning and start worrying again! But I can't actually get up and do the stuff, because I'm a) too exhausted and b) not in the mood for DD to wake up screaming blue murder because I dared get out of bed (a recent charming habit of hers).
Add to that all the times I can't sleep because of heartburn, and late nights for various reasons, and DD waking us up in the middle of the night... and yep. Not fun.