Courtesy of Miguel Angel Bermudez
It is generally a good idea to always use realistic emitter intensities for your emitters. Easiest way is using the emitter presets such as the incandescent 60W light bulb preset. If you feel the emitters in your scene don't emit enough light to properly expose the render, it is tempting to raise the emitter intensity unrealistically to compensate, but very high emitter intensities may result in more noise and "spots". Instead you should change the camera settings that affect exposure (f-Stop, Shutter Speed, ISO).
For example if you are rendering with Physical Sky, then switch it off and want to do a night time render with only emitters, it is normal for the render to look very dark if you don't change the camera settings. Daylight is MUCH stronger than normal indoor/outdoor lighting and in the real world you would have to use very different exposure settings.
As a general rule, to have a properly exposed typical indoor render lit only with artificial light the camera settings would be: f-Stop 2.8, Shutter Speed 1/30th of a second, ISO 400.