Do People Who Live in Polar Night Sleep Worse? Two Norwegian Studies Say Barely

Vidje Hansen, a professor at the University of Tromsø, told Science Norway in 2011 that he put little stock in winter depression but did believe in seasonal sleep variation. The study he was asked about found no sleep variation at all, to the surprise of its own authors, and its lead author, Børge Sivertsen, said the team saw no seasonal trend whatsoever.

The short answer to the question in the headline is that in two large Norwegian population studies sleep duration did not change with the seasons, and insomnia changed only a little. Roughly 50,000 adults in Nord-Trøndelag (63° to 65° north) reported about eight hours in bed year round, and 21,083 adults aged 40 and over in Tromsø (about 69.6° north, where the sun stays below the horizon from roughly late November to mid-January) showed no seasonal or monthly effect on sleep duration, with winter insomnia slightly more common among men only.

What did the Nord-Trøndelag study find?

The first study used the HUNT survey from 1995 to 1997, covering nearly 50,000 people in Nord-Trøndelag County, and was published in the American Journal of Epidemiology in 2011 by Sivertsen, Simon Øverland, Steinar Krokstad and Arnstein Mykletun. It found no monthly variation in reported insomnia or in time spent in bed, which averaged about eight hours a day year round.

Sivertsen told Science Norway the team was surprised and unsure how to interpret the finding. They had expected more sleep problems in winter and less time in bed in summer.

Does the far north behave differently?

That was Hansen’s point. Nord-Trøndelag lies south of the Arctic Circle, so it never gets the extended darkness of places further north, and Sivertsen himself said results could differ there.

The Tromsø Study took it up nine years later. Sivertsen, Friborg, Pallesen, Vedaa and Hopstock analyzed the 2015 to 2016 survey wave, a population-based sample of citizens aged 40 and older: 21,083 participants, a participation rate of 64.7%. Their paper in Chronobiology International (2020) reports no seasonal or monthly effects on sleep duration. Insomnia was most common in the winter months among men but not women. Time to fall asleep was slightly longer in winter, though the authors called the differences small and hardly of any clinical relevance.

Why would light matter less than expected?

The authors’ guess is the lamp. Tromsø, they note, is a modern city with considerable artificial light, which may contribute to stable sleep through the year. In the 2011 interview Sivertsen described the same adaptation: indoor lighting in winter and blinds or curtains in summer.

For the reverse question, what happens to sleep when a population gains electric light, see the four-study review of electric light and sleep length.

Could the way people were asked explain the result?

Possibly, and the researchers say so. Sivertsen noted that earlier studies often asked direct questions about seasonal effects, which may have led participants to agree that winter was worse. The HUNT survey did not tell participants what was being measured. Hansen agreed this was unusual, but cautioned that a fully blind design might show smaller differences than his own studies had found, a point worth keeping when reading any seasonal claim. Sivertsen added that positive findings are more likely to be published, so a null result in a large sample deserves some weight.

Reading any such paper for this kind of issue is the subject of a checklist for reading a sleep study.

What does shift in the polar night?

Sleep length held steady in both datasets. The alarm clock’s job did not. Where the sun gives no cue for roughly seven weeks, the morning has to be set by something else: the clock on the wall, the school bell, a person expecting you. The same reasoning applies at a smaller scale to waking before sunrise in winter anywhere, which the question-and-answer on seasonal low mood and wake-up time covers. Whether any outside cue helps in the polar night is a guess, since neither study measured wake-up methods.

What the studies leave out

Both rely on self-reported sleep. Both are surveys of adults, and the Tromsø sample starts at 40. The HUNT data were collected in 1995 to 1997. Neither paper measured individual light exposure, so the artificial-light explanation is a hypothesis, not a result.

If you came for a number

Does sleep duration change between summer and winter in northern Norway? Not in the two large population studies. The HUNT analysis (Sivertsen, Øverland, Krokstad and Mykletun, American Journal of Epidemiology, 2011) found people spent about eight hours in bed year round with no monthly variation. The Tromsø Study analysis (Sivertsen, Friborg, Pallesen, Vedaa and Hopstock, Chronobiology International, 2020), with 21,083 adults aged 40 and over (64.7% participation), found no seasonal or monthly effect on sleep duration.

Is insomnia more common in the polar night? Slightly, and only for some. In the Tromsø Study data insomnia was most common in winter among men but not women, and the time taken to fall asleep was a little longer in winter. The authors judged the differences small and hardly of clinical relevance. Women showed no winter rise in insomnia.

Why might the effect of darkness on sleep be smaller than expected? The researchers point to artificial light. Tromsø is a modern city with a considerable amount of artificial light, which the authors suggest may help explain the stable sleep patterns across the year. Sivertsen has also said people adapt with indoor lighting in winter and blinds in summer. Neither paper measured individual light exposure, so this remains an explanation, not a result.

Do these studies prove darkness does not affect sleep? No. Both rely on self-reported sleep in adults, and one covers a region south of the Arctic Circle. They show that average sleep length and insomnia did not swing with the seasons in these populations, which is narrower than showing light has no effect.