Use one large watt light bulb instead of two or more low watt bulbs. One 100-watt light bulb uses less energy and gives off more light than two 50-watt bulb. Avoid halogen floor lamps that consume 300W or more power and wastes electricity through heat. Choose light, reflective colors for room ceilings and walls.
Hereof, how much does it cost to leave the lights on all day?
A “normal” incandescent bulb costs about 0.75cents per hour, and LEDs or CFLs cost only one-sixth of that – so leaving the lights on (either overnight or while you're at work for the day, say both are about 8 hours) costs you roughly 6 cents for a normal light and a bit over 1 cent for modern bulbs.
Can you start a fire by turning the lights on and off?
Turning a fluorescent light on and off rapidly will shorten its life, though, and turning the switch on and off can wear out the switch. Perhaps this is where the idea that you can cause a fire by continuing to switch the lights on and off.