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The following is taken from the book "Greenhouse tomatoes,Lettuce & Cucumbers" from the folks at Michigan State University.

Tomato plants started during the late fall or winter months should be exposed to as much light as possible during normal daylight hours. Artificial lights, if used, should be employed only during the daylight hours on dark cloudy days, and should not be used at night to extend the length of day.
The tomato is a facultative short day plant which flowers and fruits earliest if the day is not extended beyond twelve hours by artificial light. Young tomato plants do not need the light intensities of full sunlight.
If days are short (nine hours of light), the tomato plant will flower earlier and form the first cluster after fewer leaves than if day lengths are twelve or eighteen hours. An increase in light intensity also hastens flowering.


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