That's a new one on me too. My first guess would be that (any) glasses might reduce airflow around the eyesocket, thereby reducing heat loss (would've never guessed "perceptible difference" though).

As for sunglasses, I thought that only mirrored sunglasses reflected heat to any real extent. Wrong again.

Of course, the downside is that while you're trying to hike out at night (using your GPS or compass) with Oakleys donned, you can't see the 100-ft. cliff comin' and you fall to your death ? but with warm eyeballs to the bitter end. <img src="/images/graemlins/grin.gif" alt="" />