I think it has to do with vampires, hence the importance of 30 “days” of darkness.

Pete

plot:...isolated town of Barrow, Alaska, in the extreme northern hemisphere, which is plunged into complete darkness annually for an entire month. When most of the inhabitants head south for the winter, a mysterious group of strangers appear: bloodthirsty vampires, ready to take advantage of the uninterrupted darkness to feed on the town's residents. As the night wears on, Barrow's Sheriff Eben (Josh Hartnett), his estranged wife Stella (Melissa George), and an ever-shrinking group of survivors must do anything they can to last until daylight.