I had noticed it actually. I've always had slightly discoloured teeth and having them whitened a few years ago didn't really make a great deal of difference.
I don't think this is just a case of the dentist trying to get me to pay for whitening, as firstly she said it might not make any difference (as it wasn't due to food/tobacco/wine/coffee/staining) and secondly it's an NHS practice and they don't even offer whitening, meaning I'd be going elsewhere to get it done anyway.
And she said it's not a case of me being adequately internally hydrated by drinking enough. It's that the teeth are used to being externally hydrated by saliva and get dried out when breathing through your mouth for extended periods.
Mark Neeld, thanks for your input. I'm going to go back and ask her again for further explanation and tell her that a couple of other dentists I mentioned it to have never heard of it. I don't think she's simply trying to extract dosh from me, so I'd like to know what she's talking about. I assume she isn't just making it all up for her own amusement and talking utter bollocks, so I'd like to know where she got her info from...