I am not sure on exact sciences, but I believe the consensus is to cut before or as the lights come on. Some people even go as far as to give the plants 24 or more hours of dark. Some people believe the buds push out more thc in the dark, others go on the knowledge that light degrades the thc, etc... There are many theories out there, but most people let the plants sleep before they are murdered. Seems like a nice thing to do IMO... I cut right when Lights come on, but that is mainly because that's the time I get home from work and spend time with my girls.