What is the midnight sun?

The answer

It is the summer period when the sun stays above the horizon at midnight, mainly north of the Arctic Circle. The further north you go, the longer the stretch lasts. Farther south there is no true midnight sun, but nights are short and pale, with so-called white nights when the sky never becomes fully dark. The light can disturb sleep, so a sleep mask or thick curtains are useful, and exact dates vary by latitude and year.