"The 24-hour darkness during the winter near the Poles can disrupt sleep – this is how the people who live there cope.
Imagine not seeing the Sun for weeks or even months. It's dark, the air has a freezing sharp bite and everything is covered with thick snow.
But even in the midst of the polar night, it isn't pitch black. At times the rays of the Sun filter weakly through the upper layers of the atmosphere, creating twilight shades of blue, pink and purple.
There's also light from the Moon and the stars. The northern lights make a frequent appearance and the snow reflects all artificial light, creating a "snowglow" that makes everything look brighter.
It is a stunningly beautiful time of year for those living in the Arctic. For locals, the polar night is part of life, and many thrive, embracing the season. Some even say they sleep better than they do at other times of the year.
"Polar night is too short," says Esther Berelowitsch, 42, who lives in Inari, Finland, where the polar night lasts for six weeks. "I'd love if it lasted for two months. Nature shows itself even when there's no sunlight... I go to bed earlier and sleep better. I don't want spring to come."
How you think about the winter months may be an important factor here.
"Cold and darkness affects all of us," says one of the study's authors, Kari Leibowitz, a psychologist who researches people's wellbeing in the winter. "What makes a difference is how we respond to them emotionally and behaviourally."
The study found that people who look forward to enjoying the opportunities that winter brings, such as skiing and cozy nights spent with loved ones in front of a fire, reported better winter wellbeing.
For Michèle Noach, an artist who shares her time between the UK and Vadsø, a town in the far north-east of Norway close to the Russian border, the polar night induces a feeling of coziness where she just wants to curl up and snooze.
But Leibowitz thinks something else might be going on too.
"Those with a negative mindset are fighting against winter," says Leibowitz. "For example they put on big lights to push away the darkness. But that just makes things worse: they create a contrast that makes the outside world even darker – and overlighting may also disrupt their sleep."
Håkan Långstedt, an engineer based in Helsinki, Finland, and managing director of lighting design company SAAS Instruments, recommends soft lighting during the polar night.
"If you have a lot of darkness, you don't need a lot of light to compensate," he says. "You only need low-level lighting."
Leibowitz, who suffers from a sleep disorder herself, also recommends using soft lighting and natural light, such as candlelight and a fire, in the hours before going to bed to improve the quality of sleep.
Esther Berelowitsch, who moved to Inari in northern Lapland from Paris eight years ago to learn Sámi languages, spends two hours a day walking or skiing outside the house during the polar night.
"It's important to be outdoors when we have a little light," she says. "I only stay indoors when it's -40C (-40F) or colder. Otherwise I go out for a walk or go skiing."
"What we see is that the sleep-wake cycle is delayed in winter," says Arne Lowden, an associate professor who studies sleep and stress at the University of Stockholm, Sweden. "People go to bed later and if they have to wake up at a certain hour because they have to go to work, their sleep will be cut short," he says.
"Cold temperatures and fewer daylight hours disrupt the circadian rhythm, that is, our body clock," Lowden says. "Our bodies operate a 24-hour cycle in which to perform their functions. Almost all our organs are organised in cycles, in a way that one time of the day they're active, and another time they're recuperating and restoring cells."
"If your circadian rhythm is disrupted, you'll be sleepy during the day," says Lowden. "If it's badly disrupted, you'll wake up at totally the wrong time and you won't be able to hold a job."
"We've seen a change in users' resting heart rates as well: they increase by 3% from summer to winter," says Heli Koskimäki, head of future physiology at Finland-based technology company Oura Health. He confirmed that most of the users were in the Nordic countries, Canada and the US, but was unable to say how many lived in places that experienced the polar night.
Koskimäki says users who follow a daily schedule according to their chronotype seem to sleep better. Put simply, this means getting up early in the morning and going to bed early in the evening if you're an early bird, and getting up and going to bed later if you're a night owl.
Modern society tends to favour morning types, says Koskimäki. "If you're more of an evening type, there's a danger that you'll sacrifice some of your sleep to meet society's demands. If you can't follow your own chronotype, try to be at least as consistent as possible with your sleep schedule: you'll sleep better and feel better for it."
Taken together, these findings suggest that sticking to the exact same daily schedule throughout the year may not be the best idea.
"If you have flexibility in your private life and in your job, you're better off if you adapt your schedules to the seasons and give yourself more time to sleep in the winter than the summer," says Leibowitz.
Berelowitsch, who loves the polar night, says it gives her an opportunity to slow down and engage in quiet pursuits.
"I've noticed that in my free time I do everything slower – I walk slow, eat slow, sleep slow," she says. "When it's too cold to go out, I remember my handicraft projects and my guitar. But I also enjoy socialising on dark winter evenings."
"Polar night is a time for gathering with family and friends," agrees Lowden.
Noach says there have been times when she has sat with friends at the fireplace for nine hours, unaware of the passing time. Polar night also gives Noach a unique opportunity to absorb herself in art. "For two months, everything's happening in the dark," she says. "I find that I'm very creative then. I'm mining myself.
"In the Arctic they say during polar night you have to switch your internal lights on," she adds. "So you have to draw on very deep resources in yourself. There are people who find it difficult and depressing. But the majority up here get a real kick out if it."