Under tank heaters are a much better source of constant, even heat than any kind of overhead lighting, and any kind of light left on 24/7 will be stressful to the snake. The best thing would be to switch to a UTH (regulated with a thermostat, of course). If you cannot do this, at least get either a ceramic heat emitter or unfrared/moonlight type bulb that will not interfere with their day/night schedule.

Depending on what type of snake you have, some snakes can get very ill if their temps are allowed to even drop into the 70's every night. Respiratory infections and regurges can be brought on by temps this low in some species.