3R Lighting/ tutorial: How many hours a day to burn a grow light

How many hours a day should a grow light burn?

Q: How many hours a day should I burn my lights?
A: There is no one best way to light plants:
It depends on the type of plant, the kind of plant (species), and exactly what you want the plant to do.

The typical rule of thumb for flowering plants is to run a 16 to 18 hour light cycle then 6 to 8 hours of dark time, during the vegetative stage. Then at the Bloom stage, switch to 12 on/12 off. It is also typical to run a Metal Halide during the Vegetative stage then switch to a High Pressure Sodium Light during the Bloom stage.

Q: What happens if the lights burn 24 hours a day?
A: The plants will grow.
After about 16 or 18 hours in most plants, though, growth levels off. So if you turn off the lights for a few hours, you can save some electricity and not really affect growth.
Plants often use dark time to build up Florins which are hormones plants use to bloom. This is why the light is often cut back during Bloom stage.

Q: If the Florin hormones need dark time to build up, can I burn a High Pressure Sodium Light 24 hours a day to keep lettuce from bolting and going to seed?
A: No., we've tried it, but it is another great theory shot down by the facts.