I was a submariner. I started taking vitamin D after a psychologist gave my division a speech about her experiences with treating veterans who had alarmingly low vitamin D levels. The next time I went underway, I brought a bottle of 5000iu supplements with me, and it was a night and day difference. I started to feel like normal and part of a team, and not like I was in prison. I work nights and still take them sometimes, especially if I start to feel signs of seasonal depression or general depression symptoms.