Your indoor enclosure should be as close to natural as you can make it. That means, that the light (sun) would get turned on when the sun comes up, and get turned off when the sun goes down. If its too cold in your house at night without the light, then you need to find a heat source that does NOT provide light...ceramic heat emitter, black or red light, etc.
missgemma, do you know what the temp is at night in the enclosure where you are keeping 'Trevey"? These are some things you want to know; What the temps in your enclosure is under the basking lamp, in the cool area and in his hide. You will want to know the humidity in those areas also so you will need a thermometer, hydrometer. Here is what your temps and humidity should be.
Daytime Air Temperature: 70-90°
Nighttime Air Temperature: 60-65°
Basking Temperature: 90-95°
Humidity: 30-50%