Running lights around the clock in veg is tempting for sativa seedlings because they respond so quickly to it, but the plants seem to do just as well on eighteen hours with a rest period. The extra six hours of light adds cost without much extra growth on these lines, and some growers report that the dark period helps keep the stretch in check. A side by side veg test on https://hazesativaseeds.com found almost no difference in node count between the two schedules over four weeks. I have settled on eighteen six and never looked back.