North of the Arctic Circle the sun refuses to set for part of the summer and refuses to rise for part of the winter. Sweden has a good deal of country up there, and the effect is far stranger than the phrase "long summer nights" suggests.
The Earth's axis is tilted about 23.4 degrees relative to its orbit. That tilt is what gives us seasons: for half the year the northern hemisphere leans towards the Sun, for the other half away from it.
Near the equator the effect is mild and day length barely changes. The further from the equator, the more extreme it gets, until at 66.5 degrees north the lean is enough that on midsummer day the Sun never dips below the horizon at all. That latitude is the Arctic Circle, and it exists for exactly that reason.
The Arctic Circle crosses Sweden just north of Arjeplog and runs straight through Jokkmokk, which is why the town has a marker for it. Everything north of that, including Gällivare, Kiruna, Abisko and Jukkasjärvi, gets both phenomena.
The length depends entirely on how far north you stand:
The Arctic Circle is not quite a fixed line, incidentally. The Earth's tilt varies slightly over millennia, and the circle currently drifts north by about 15 metres a year.
The Sun does not hang overhead at midnight. It sinks towards the northern horizon, skims along it, and climbs again without touching. The light at that hour is low, gold and sideways, the golden hour stretched across several hours instead of one.
The practical difficulty is sleep. Without darkness the body loses its main cue for when night is, and most visitors need curtains and a certain stubbornness. Locals will tell you the first summer is the hard one.
The winter counterpart has a worse reputation than it deserves. During polar night the Sun stays below the horizon, but only just below, and it still lights the sky from underneath.
Around the middle of the day the light climbs to a deep blue twilight lasting several hours. It is enough to see by, and snow reflects most of it back, so the landscape is often brighter than a cloudy November afternoon in Stockholm. The Finnish word kaamos captures it better than any Swedish one: not black, but blue.
Polar night also brings the best of the aurora season, since darkness is the one thing it requires. We cover that in northern lights in Sweden.
You would expect the midnight sun and the polar night to balance out exactly. They do not: the midnight sun lasts noticeably longer, by something like a week at a given latitude.
Two effects cause this, and both work the same way. First, the atmosphere refracts sunlight, bending it around the curve of the Earth, so the Sun is visible when it is geometrically already below the horizon by about half a degree. Second, sunrise and sunset are defined by the upper edge of the disc rather than its centre, which buys another quarter of a degree or so.
Both push the balance towards more sun and less night, everywhere on Earth. It is also why day and night are not exactly equal at the equinoxes.
You do not need the Arctic Circle for strange summer light. From late May to mid July, most of Sweden never reaches astronomical darkness at all: the Sun stays close enough to the horizon that the sky keeps a blue cast all night.
That is why stargazing effectively closes for the summer, and why a June night in Stockholm feels less like night than like a very long dusk that turns back into dawn.
Nollan.se shows sunrise, sunset and the golden hour for whichever town you choose, which is the quickest way to see how extreme the swing is where you live.
At about 66.5 degrees north, which in Sweden means just north of Arjeplog and straight through Jokkmokk. North of that line the sun stays above the horizon for at least one full day each summer, and stays below it for at least one full day each winter.
Roughly from the end of May to the middle of July, about seven weeks. The further north you go the longer it lasts, and at the Arctic Circle itself it is down to a single day around midsummer.
No, and this surprises most first-time visitors. The sun stays below the horizon but not far below, so the middle of the day brings several hours of deep blue twilight, often bright enough to walk in. Snow reflects it, and a moonlit day can be genuinely bright.
No, not the true midnight sun with the disc above the horizon. But in June the nights around Stockholm never get properly dark, only a prolonged blue twilight, which is why the aurora and most stars are invisible even on a clear June night.
Two reasons. The atmosphere bends light, so the sun appears above the horizon when it is geometrically just below it, and the sun is a disc rather than a point, so its upper edge shows before its centre. Both effects push in the same direction and add roughly a week.