Most energy-saving light bulbs work the same way as the old long tube type, but with everything made smaller. When electrons flow through the gases in the tube, mercury gas inside it gives out ultraviolet light. The white coating in the glass absorbs this light and emits it again as light you can see. The electrodes in both ends of the tube have to be warm before electrons can travel across it, and this may take some time on older light bulbs. At low temperatures, mercury condenses into droplets and the bulb will also need to, quite literally, ‘warm up’ before getting properly bright.
Answered by José Monteiro, Science Museum