Highest suicide rates occur during the spring, around the vernal equinox not around the darkest time of the year. If you've never experienced the northern spring where each day is 10+ minutes longer than the previous, you'll never understand it. It just fucks up your mind in ways that are hard to describe.
My kitchen psychology thinks the high suicide rates in the spring may be related to the fact that people around you start to get more positive and active (some too much so) but if you're suffering from a bad depression, seeing that around you will make it worse.
It's not just suicides that peak. It also affects breakup/divorce rates as well as forming new relationships. It probably affects professional careers too, but I don't know if there are stats about it.
The long, dark winters and the rapid change that follows brings out some very primal sides. That being said, I'd recommend travel to the extreme latitudes around the solstices, both of them.
side note: we have about 15 hour long days at the moment. It's still pretty dark at night but I can't wait for the summer when you can see the sunset and sunrise at the same time if you look to the north at around midnight... it's magical.