This is from High Times the closest thing we have to a peer reviewed research article.
What Time of Day to Harvest?
Timing the harvest is
Paramount to the final quality. Harvest your precious buds in the dark, just before the lights normally come on. If possible, do not allow the plants to see direct light as long as their roots are attached. Direct light on a plant will draw up stored starches and sugars from the root system.
During the nighttime hours, our ladies are busy storing food down in their root system that they made during the daylight hours. During “lights out," starches and sugars produced by photosynthesis during the day drain downward to the roots. Knowing this, it is easy to figure out that you want to cut your plants away from the roots
before the lights come on, when food moves back upward into the buds.