Kaamos explained: where the sun really stops rising, what bright light does to winter depression, and the one claim that rests on 46 people.
What is kaamos? Kaamos, or polar night, is the winter period during which the sun does not rise above the horizon at all within a twenty-four hour day. It is an astronomical condition, not a description of gloomy weather, and it has a precise start and end date for each location.
Does the polar night happen everywhere above the Arctic Circle? Not quite, and this is the part that is usually got wrong. The Arctic Circle marks where at least one polar night day would occur, but the atmosphere bends light and keeps the sun visible from slightly below the horizon. In Finland the real boundary sits somewhat north of the Arctic Circle line.
How long does the polar night last? It depends entirely on latitude. At Nuorgam in Utsjoki, the northernmost point in Finland, the sun stays below the horizon from roughly 24 to 25 November until 17 to 18 January, which is more than fifty days. For comparison, Longyearbyen on Svalbard runs from about 26 October to 15 February, over 110 days. We give no figure for Rovaniemi or Sodankylä because the source we read does not state one.
Is it completely dark all day? No. The sun does not clear the horizon, but it comes close enough around midday to light the sky, and snow returns most of what light there is. The middle of the day reads as a long blue twilight rather than as night.
Does a light lamp help? For treating an episode of seasonal affective disorder, bright white light has a real evidence base: seventeen randomised trials and 773 participants, with white light outperforming placebo and outperforming blue, green and red. The trials that worked used around 10,000 lux for thirty minutes to two hours a day. For preventing an episode before it starts, the evidence is one trial with 46 people, rated very low quality, from which the reviewers said no conclusions could be drawn. Those are two different questions with two very different answers.
Are blue daylight lamps better? The trials do not support that. Blue light ranked third of four colours, behind white and green, and in the stronger blue regimens one third of participants reported headaches. One caveat in fairness: blue light was tested at under 200 lux in all but one study, while white light was usually tested at 10,000 lux, so the comparison is not like for like.
Does the sauna or ice swimming help with winter depression? We have no evidence for that and we are not going to imply it. Sauna and cold water are how a dark evening is structured here, which is a cultural fact rather than a treatment. Seasonal affective disorder is a recognised diagnosis, and the person to talk to about it is a doctor.
For part of every winter the sun does not clear the horizon at all. That is kaamos, and it is not the same thing as the long dark evenings further south. It is a measurable astronomical condition with a start date and an end date, and the thing people reach for to get through it has a research record worth reading before you buy anything.
The Arctic Circle is not the kaamos line.
Kaamos, also called polar night, is the winter period during which the sun does not rise above the horizon at any point in the twenty-four hours. It happens only above the Arctic Circle, but the Arctic Circle itself is not where it starts. Because the atmosphere bends light, the sun is still visible from below the horizon for a while, and the real boundary in Finland sits somewhat further north.
At Nuorgam in Utsjoki, Finland's northernmost point, the sun stays down from roughly 24 to 25 November until 17 to 18 January, which is over fifty days. We give no figure for Rovaniemi or Sodankylä here, because the source we read does not give one and we are not going to calculate it ourselves.
Kaamos is also not total darkness. Around the middle of the day the sky goes blue and the snow carries what light there is, which is why photographs from the period look lit even though the sun never appeared.