Thanks...I think?

Last night someone said to me "how are you doing, you look tired." Now what in the world is the appropriate response to that comment? Are you supposed to say "thank you?" I was just thinking to myself "wow, thanks for pointing out that I naturally look fatigued". Of course, I didn't say that. And really, I didn't have an excuse for looking tired - we got one whole extra hour of sleep courtesy of Daylight Savings Time AND I got a nap in the afternoon! I also could have said "thanks for noticing my genetic disposition to under eye bagginess", but that wouldn't be appropriate either. Now I'm sure that I have probably made this comment to someone in my lifetime, and I know for a fact that this isn't the first time someone has said it to me. There is a part of me that wants to sting them back and say "yeah, and your hair looks a little frizzy today too" - but that wouldn't be very polite, now would it? :) Being the ever-polite girl that I am I simply said "oh, well I suppose that's the permanent state of motherhood, huh?" I'm open to ideas, as I'm sure this won't be my last encounter with the you-look-tired comment.