Tungsten Filament Light Bulbs
The tungsten filament light bulb design as we know it, was invented around 100 years ago. It was designed to last as long as possible and to be cheap to manufacture, I doubt very much consideration was made to how much electricity it used or heat it generated.
The tungsten filament bulb generates a lot of heat, which most of the time is wasted. There is an argument to say that this heat is useful in heating the home. Even if the room could do with additional heating, a light bulb isn't a particularly good way to go about it. Most of the time the filament bulb will be near the ceiling and because hot air rises, it's just heating the warmest part of the room anyway. Of course, this means even on a very hot night the bulb will still be producing heat and adding to the problem of the room being too warm.
Low Energy Light Bulbs
Compact fluorescent light bulbs generate very little heat which means they're far more energy efficient than the older filament bulb. They have also dropped in price so much in recent years that it makes no sense not to buy compact fluorescent bulbs to replace your 100w bulb with an 11w energy efficient bulb instead.
There are however some downsides to fluorescent bulbs. These energy efficient bulbs do contain a small amount of mercury. There is a small risk of mercury poisoning if the fluorescent bulb is smashed, but as long as the area is thoroughly cleaned there is little to worry about.
Due to the mercury content of compact fluorescent bulbs you can't just throw them into the bin at the end of their life. The bulbs should be taken to hazardous waste collection centre so that they can be carefully recycled.
Dimmer Switches and Fluorescent Lights
One downside to compact fluorescent light bulbs is the problem of dimmer switches. You can't normally use an energy efficient light bulb with a dimmer as it will just cause the bulb to flicker and reduce the life of the bulb.
Fortunately however, a number of companies have been working on this problem and there are a few efficient light bulb already available. The Osram Dulux El Dim bulb was one of the first which is dimmer switch compatible, but other solutions are available.