[Tomaddict]

Too much light is not helpful. During periods of adequate light, most of a plant’s energy is focused on manufacture of carbohydrates necessary to fuel cell growth; there is a little, but not a great deal, of cell division and growth during light periods. Those accumulated carbs are what fuels growth of the plant. During dark periods, plants devote the great majority of their energy expenditure to cell division and cell growth. An endless supply of light will eventually disrupt the balance between carbohydrate accumulation and utilization, as there is a limit to how much carbohydrate a young plant can accumulate.

The relative length of day and night is also a key determinant for the beginning of the fruiting stage for many plants, but I don’t believe this is true for tomatoes – fruiting (beginning with flower formation) appears to begin when plants have developed a sufficient carbohydrate supply, or sometimes when they undergo certain stresses (such as irregular water supply).

And remember, plants are adapted over many generations to what mother nature provides. It might be helpful to extend the period of light modestly, but i'm confident that it will be better to give them several hours of dark to grow and reset for the next photoperiod than to give them endless light.