The Nordic countries are no strangers to the long, dark winter. Despite little to no daylight — plus months of frigid temperatures — people who live in northern Europe and above the Arctic Circle have learned how to cope mentally and physically with the annual onset of the winter blues, which can begin as early as October and last into April for some. The winter solstice will occur Dec. 21, marking the shortest day and longest night of the year in the Northern Hemisphere. While sunlight be...