Here's the little snippit of info I found:

"The Stichting Institute of Medical Marijuana (SIMM), the first company to sell marijuana through the pharmacies of Holland, has been investigating the medical possibilities of cannabis, together with TNO laboratories and the University of Leiden. One of their discoveries has been that to keep the ripe plants in the dark before harvesting could increase their potency.

SIMM’s growers separated a crop of mature plants, harvested half of them and kept the other half in absolute darkness for 72 hours before cutting and drying. Analysis of the resulting dried buds showed that some varieties had seen an increase of THC of up to 30%, while CBD and CBN remained the same."