If I switch the light on and off every time I enter and leave a room, does this use more energy than leaving it on all evening?

Switching the light on and off does saves energy, but there is a catch. Every time you flip the switch, the bulb takes a jolt of electricity, which shortens its life. Studies by the Electric Power Research Institute in Palo Alto, California, found that turning low-energy compact fluorescent bulbs on and off at frequent intervals can shorten their lifespan by as much as 75 per cent. The institute's director of energy utilisation, Tom Reddoch, suggests leaving energy-saving bulbs on if you will be out of the room for less than 15 minutes. (New Scientist)

Now, the above questions seems to be oblivious to the urban myth that lights use less electricity if just left on all the time. This myth ignores that this came from fluorescent lights, where the initial charge needed to create the plasma in the tube is high compared to that needed to keep it going. So, switching a fluorescent on-off-on-off like a manic child is just dumb, but it has now become a general urban legend unplugged from its source. The mental energy needed to stand there ruminating over whether to switch the light off or not is far greater than the energy used by the light. Whichever you decide will be both right and wrong at the same time as judged by the same people. To save your sanity just do what you feel is right. If you leave the lights on after falling asleep you should be punished by being made to plant a tree.