The "cooking" will depend upon the lights.. some let off a lot of heat, while others very little.
As mentioned before, they do need a day/night schedule.. usually 10-12 hrs is what is recommended for keeping the light on. It will also help make sure algae won't go crazy in the tank - if it does then a little less light would be good.
Don't worry about natural light, or light from the room going into the tank past the 10-12 hr mark - as long as it gets dim in the tank, they will find rest.
I turn my tank lights on around 7am, and off around 7pm - but the room light is on for hours after with no issues from the fish.
The "fish room" lights are off and on throughout the day/night, as my boyfriend is sometimes up all night, etc and have had no problems with any of the fish in that room that has to do with stress, etc. Fish do adapt to routines and will sleep during the day if needed. Why sometimes having a cave is good for them to be able to go into a dark place and rest if they wish.
The only time having a light on 24hrs is when males are caring for their eggs - some breeders will keep the tank lights on, or have ambient light nearby to help him out. But that is only for a few days..