Do not exceed 37c. In fact i think that 35 may be a little to hot. I actually lost a frog when i left town for a few days we had a power glich so my timer didnt shut of when it should have the temp of the tank was around 40c when i came home my frog died 45 minutes later. My bulb was a 150w. that only stayed on for 4 hours then was off for 4 hours. i had that at a continuous cycle. there are care sheets on this site that will give you exact temps. I am just surprised that a 40 watt bulb is producing that much heat. Is your tank ventalated?