Why do those in Alaska experience 80 days of solid sunlight in the summer

Alaska is called as mid night sun because of it's position in Arctic Circle, latitude 66°33' north.

Explanation:

When the sun is located above the horizon for a full 24 hours, it is called as summer solstice and when it is located below the the horizon for a full 24 hours, it is called winter Solstice. In Alaska, sun travels at 360° direction in the sky thus Alaska receives 24 hour of light during summer time.