Idiots guide to grow lights

Are any of you leaving them on all day and all night? I understand the plants don't care whether they have a nighttime or not.

Plants need some darkness every 24 hours - they've evolved to work with this daily cycle. They photosynthesise (using up carbon dioxide and producing oxygen) when it's light, but when it's dark they "breathe" like us, taking in oxygen and producing carbon dioxide. So if you keep them under lights 24/7, they'll use up more carbon dioxide and thus be less able to photosynthesise, especially if they're in an enclosed propagator.

I'm running my fluorescent lights for 16 hours a day at the moment (4am - 8pm), but I'll turn them off when the days get a bit longer and warmer - by then there won't be space in the propagator for them anyway!