Polarnatt means 'polar night', and is used to describe a period of time when the sun sets for more than 24 hours.
It occurs in the northernmost and southernmost parts of the earth, within the 'polar circles' (the Arctic and Antarctic Circles), hence the name. That includes parts of Swedish Lapland, with Kiruna being the largest town within the Arctic Circle. There, the polar night lasts for almost a month.
The polarnatt is the opposite to the midnattssol (midnight sun, also called the polar day), which is when the sun doesn't set for at least 24 hours. And another name for the polar night is middagsmörker or 'midday darkness'.
Because of the way that the sun's rays are bent, the midnight sun lasts for longer than the polar night; in Kiruna, the midnight sun lasts for 50 days compared to 28 of polar night.
But even during polarnatt, it's not necessarily totally pitch black.
That's because the requirement for the polar night is that the sun doesn't rise above the horizon, so it may still be level with the horizon – so-called polar twilight. This usually means deep blue skies with pinkish hues which are popular with photographers, and clear skies combined with polarnatt tend to offer some of the best chances to see the Northern Lights.
White snow and moonlight also add to the lightness.
However, the polar twilight isn't much use to people who find their moods affected by lack of daylight, since the twilight is too low a level of ambient light to offer the psychological benefits of sunlight.
Polarnatten i Kiruna är en tid av mörker, men det kan vara mysigt
The polar night in Kiruna is a time of darkness, but it can be cosy
Polarnatten sveper in över norra Sveriga
The polar night is sweeping in over northern Sweden