Whether you use 18/6 18 hours of light and 6 hours of darkness or 16/8 it is a matter of preference and would have to be tested to see what results you may have. I read a study where lighting schedule versus root length was tested and plants grown under a 16/8 lighting schedule grew the longest roots, 18/6 was not tested. I was made aware of a study where lighting schedule versus growth was tested and plants given at least 5.5 hours of darkness grew the most.