Lights will sometimes dim for a few seconds and then come back to complete brightness again. This can happen when a light is connected to the same wires that provide power to an appliance that takes a lot of power, like a refrigerator, a microwave oven, or an air conditioner. The reason the light dims for a few seconds is that the appliance is using a lot of power when it first starts up. After the appliance is running for a few seconds, it will use less power, and the light will return to normal again. If you have central air-conditioning, the lights may dim each time the air conditioning comes on.
You will usually notice this dimming more at night (for obvious reasons!), but you might also notice it in the daytime. If this dimming bothers you, you can handle the problem by having an electrician add another circuit specifically for the appliance that is causing the dimming problem.
NOTE: If you haven’t changed anything electrical in your home or office, and you suddenly start to have dimming problems or power fluctuations, then you probably have a loose wire somewhere. You should contact an electrician skilled in troubleshooting to find and correct this problem.