"One question I have been
asked repeatedly by medical marijuana growers is when to harvest in relation to the light cycle. Many growers want to know if harvesting during the light cycle versus the dark cycle makes any difference in the potency of the cannabinoids. There have been many opinions on this subject and also many perpetuated myths. I decided to experiment with harvesting at different periods throughout the light cycle to see if there was a noticeable (measurable) difference in cannabinoid potency.
I decided to harvest three identical plants, all grown in the same room, at different times throughout the room’s light cycle. The first plant was harvested right before the light cycle occurred. The second was harvested right after the light cycle finished. The final plant was moved to a completely dark, temperature and humidity controlled room where it sat for 48 hours and was then harvested.
There was no noticeable difference in cannabinoid potency. We tested three samples from each plant and averaged the results of each plant to compare to one another. The results were virtually identical. There was no difference in potency in relation to the time at which the plants were harvested. Since machines are not always accurate, we also provided a few patients with blind samples to see if they noticed any differences in the samples. No differences were noted.
It is true that resin production on a medical marijuana plant peaks during the dark cycle. Because of this fact, there has been a myth perpetuated that cannabis harvested right before the light cycle begins — or cannabis given 24-72 hours of dark before harvesting — will produce a higher potency medicine than medical cannabis harvested during the light cycle. The problem with this theory is that a typical medical marijuana plant has a blooming period of about 50-60 days. When the overall time of the blooming cycle is taken into consideration, an additional day or two is pretty insignificant. The essential oils and resins are not produced overnight but instead are steadily built up over the entire blooming process. The number of cannabinoids degraded in one light cycle is completely negligible when comparing potency of the final product.
I tend to harvest my medical marijuana plants right before the light cycle begins, but this has always been a matter of convenience over anything else. It allows me to harvest and remove one crop and then bring in the next crop right at the beginning of the light cycle. This gives my next crop twelve hours under the blooming lights to better establish themselves before their flowering period begins (12 hours of darkness). My plants seem to transition better from vegetative to blooming if I start with the light cycle instead of immediately placing them in the dark cycle.
Harvesting during the light or dark cycle should be determined by personal preference and convenience. Growers should not get hung up on harvesting at a particular time in the light cycle. Plants harvested when they are ripe will be potent regardless of the time of day they are cut down."
http://bigbudsmag.com/grow/how/article/marijuana-harvest-myth-debunked-may-2012