How cool should night-time temps be?

During the day the warm side is at about 90-93F, with the cool side about 80-83F. At night, these drop to about 75-77F on the warm side and about 70-73F on the cool side. Does this sound ok or is it too cool at night? I have a 60W incandescent bulb on the warm side, which is on for 12 hours in the day. There is also a 13.5W heating mat under the wooden base of the viv at the warm side which is on 24 hours.

My humidity also rises dramatically at night: from around 50% during the day to nearly 80% at night. Is this just because of the cooler temps? Is this ok?

You're flirting with getting a Respiratory Infection with those night time drops. There should be no drop at night, your temps should remain constant 24 hours a day with 92-94 degrees on the warm side and 82-84 on the cool side.

Your ambient temp should NEVER be below 75 deg. ,but by just going below 80 deg. you risk a respiratory infection. I would suggest getting a night light those temps are way too low. Get a 60w black or red bulb and you should get about the same temps as you do in the day (which are just fine). The only reason you would like a night drop is if you are currently breeding your snakes....if you are not, then keep the temperature constant all the time.

Would a more powerful heat mat do the trick?