The ambient temp is quite cool. The ambient air temp inside the tub really should not be much cooler than 75 day or night and most will say 77-85 for the ambient air temps. Your cool end temp is also quite cool.

Ambient air temps go a long way to regulate core temps. regulating them is more important than most recognize. The easiest solution is to heat the room up, it will solve the ambient air cool end and you probe shift. Cool rooms can be done (I have a rack in a cool room) but it isn't easy or really recommended.