in the summer, the north part of the earth is leaning towards the sun, so Alaska(being so north) gets more sunshine. in the winter it's the opposite. that's why it's winter in the US, because the sun is farther and thus makes it colder for us. summer is hotter for the US because we are closer. south America is on opposite seasons because of this.
Alaska is dark half a year, and sunny the other half because of its position in terms of the latitude and longitude. When the North Pole leans toward the light and the Northern Hemisphere gets more light, while the than darkness, so during the course of one rotation, each hemisphere is lighted half the time.