Leaves lowering after 15h of light. Longer dark period?

Becareful of nearing 10 hours of uninterrupted darkness with any photoperiods, for future reference You can reduce length of light, dim it slightly or raise it slightly. All would reduce the amount of light any 1 spot on the plant receives per day. If you raise the light, it may increase waste, but you will regain some of that waste with better distribution of light delivered to teh entire plant, even if a bit less to the canopy and a bit more absorbed into walls or otherwise not hitting the plant. Some up and down of the leaves should occur - that is natural. I'd only react to this if it is for an extended period of time and at the end of the light cycle. You probably have a bit more light than you need for 18/6 relative to height from canopy and area of coverage (both go hand-in-hand). Read up on DLI. You'll see what you can provide is proportiona to hours of use and umol/s per m^2 applied. wether you do that over 18 hours, 20 hours or 12 hours won't matter too much. This is why "they" say you need 150% more light in bloom (for photoperiods). this directly relates to changing from 18/6 to 12/12. Two-thirds the time requires the recipricol more intesnt light.. 3/2 or 150%. Nothing mystical about it. Mileage will vary due to environmental factors and genetics, but will always be a good starting point if you understand a bit about DLI. (daily light integral)