For the winter metabolism hypothesis to be correct, sunshine should affect a person's weight. I mentioned earlier that body fat is lowest in the summer. However, we don't know for sure if people are going outside and actually getting more sun in the summer.
A different way to test this would be phototherapy. Phototherapy should help an overweight person lose weight, because it signals the body that winter is over.
I found two studies on this. The first study was only a small study, but it did generally show this result. The study showed that three of four obese patients lost weight from phototherapy.
The second study added phototherapy to a moderate exercise program. The researchers found that the addition of phototherapy had a significant effect on body fat. In fact, body fat only decreased in the exercise and light group. It did not change in the exercise only group.
