Naso Tang acting funny

[Ruth]

My Naso tang - that I have had in my 230g for 2 years now has been acting sort of funny the last few days. He still eats well and swims around. He has his favorite sleeping place - head first into a space between the live rock and the aquarium wall. He has always gone there as soon as the lights go out but lately I am finding him there during the day - almost like he is resting or something. I have never noticed him doing this before - he is usually out swimming around doing laps.
No signs of disease or anything - just the going and hiding in his spot.
Should I be worried?