Hmmm, urban legends may be a strong term but there used to be a general belief that because fluorescent lights used more power in their warm-up phase, then it was better to leave them on all the time.

This was actually never true!

According to the National Energy Foundation,

an old style strip light (or non-electronic CFL) only uses as much energy in the warm-up phase as it does whilst operating for a minute or so, and modern electronic ballast CFLs use even less energy at the start. Although CFLs do not use much electricity it is still best to keep them switched off when not wanted - why throw money away on anything that not needed?

Photo credit: King County