If you are always thirsty in the daytime, and drink diet sodas, but sleep all night, is there a medical problem or not?

Thirsty. Believe it or not, soda (including diet) is not healthy. Soda contains a lot of phosphates and sodium. So that drink that you think may be quenching your thirst may not. Caffeine, in many sodas, is a diuretic (albeit a mild one).