well i assumed the same which is why im so confused.

thats the air temperatures. My tank is wooden with a sliding glass front. So i have a uth under the substrate attached to a thermomstat. then i have a dual thermometer hygrometer attached to the centre back wall and i have two digital thermometers on top of the substrate one in the hot spot and one at the cool end. i have a heat lamp in the warm end on a 12-12 timer to raise ambient temps throughout the day. The temperature drops naturally at night but the uth is on continually so the warm end is always warm. I havent changed any of my setup apart from the substrate but this is the first time the temps have changed dramatically. Today it seems to have taken a while to warm up but now back to regular temps, im wondering if the substrate needed a couple of days to air out maybe and was affecting the readings on my thermometers? Ill see how the next couple of days go..