Although plants can't grow without light, they can't grow properly without periods of darkness, either.  Most plants respond best to normal cycles of day and night--14 to 16 hours of light and eight to ten hours of "sleep."  Sometimes varying the periods of light and dark by specific plant needs can increase growth.

It may be hard to remember to turn your grow-lights on and off at the same time every day, so invest in an inexpensive timer.  You have a choice of tabletop models or those that plug directly into the outlet.  Make certain the timer you buy has a 1,875-watt capacity.  It will then be able to carry the same load as safely as any other outlet on a 15-amp household circuit.