Since the plants are probably already starving for light, I wouldn't suggest it.

http://everybodysgardencenter.com/april/growlights.asp
Q: How many Hours should I keep my Lights on in my garden?

A: This depends on what you are growing. For flowering plants that include Roses, Tomatoes etc. 16-18 hours of light and 6 hours of darkness are needed daily for vegetative growth, and 12 hours of light combined with 12 hours of COMPLETE darkness for the fruiting and flowering phase.

Robert,

You are wrong about 12 hours of darkness for fruiting. Ever see tomatoes or peppers growing in summer? Or munch on a ripe mater in July? There are nearly 15 hours between sunrise and sunset, plus ambient light before and after.

Mike

Mike,

I want to make it clear that I'm not arguing with you and I actually value your input. It's just that I'm finding mixed evidence. So far your advice is the only source that suggests a consistent 18 hours of light. Maybe the other sources are speaking specifically for HID lights since they supply more MOLS/day than HO-FLUOROS in the same timeframe.

http://www.hydroponic-gardening.ws/grow-light-science.html

We can use blue spectrum 6500K lighting with our Hydroponic plants to create more vegetative growth in the beginning plant stages. Blue light run for about 16-18 hours would recreate the Season of spring and the production of leaves and branches.

We can then change our Hydroponic lighting to the red spectrum of 3000K for a shorter period of about 12 hours to recreate the Season of Summer. This will induce flowering and fruiting of the Hydroponic plants.

Robert

"For flowering plants that include Roses, Tomatoes etc. 16-18 hours of light and 6 hours of darkness are needed daily for vegetative growth, and 12 hours of light combined with 12 hours of COMPLETE darkness for the fruiting and flowering phase."

Roses and tomatoes are both day neutral plants, which means they flower according to maturity, or in response to environmental stimuli other than photoperiod. They have no darkness requirements for flowering or fruiting.

If you can supply their daily light requirement in 12 hours, then there's no need for more. But many light sources may require more time to produce sufficient light totals, and there is no harm in this.

Robert,

Sometimes things do not come across as they should in posts.

I re-ad yours to mean that tomatoes and such need 12 hours of darkness to fruit or flow-er, which they do not. The only thing I have re-ad from research is that after a certain point (7-8 weeks?), tomatoes and such do need at least four hours of darkness. Not so much to fruit or flow-er but to avoid other problems.

Mike