Science says at the end of dark cycle before lights hit trichs. I try to do the dark thing but if its ready its ready i wouldnt chop after hours of light beat trichs. But ive chopped day and night and its all still fire.
Dark/Light will not have as much as an affect as other things such as Drying/Curing, gentics, etc. I just chopped my first real grow and did it mid day.