So the night bulb produces u.v.? LOL what kind of night bulb is that...or are we talking about two lights?
U.V. shouldn't harm your pyxie. These frogs are diurnal and the african sun can get hot.
I wouldn't recommend a mercury vapor bulb for any amphibian, those produce A LOT of U.V.B.
If your worried about excessive heat or u.v. give your frog a shady area to retreat to.
Also if you don't already have one get yourself a thermometer or temperature gun.