Over much of winter, a dark sky sheathes Earth's northernmost cities, yielding sunlight for only a few hours a day.
These long nights and short, dim days are a result of Earth's tilted axis, causing the sun to distribute light differently across latitudes. In the summer, these northern cities experience the opposite effect: long, drawn-out days with up to 20 hours of light.
The tilted axis also means that light reaches Earth at different angular degrees. Places closer to the equator receive more direct light, where as northern countries receive more indirect light with the sun resting low in the horizon.
Five Capitals With the Least Daylight During the Winter:
1. Reykjavik, Iceland, had about 4:07 hours of sunlight on Dec. 21, 2014
2. Helsinki, Finland, had about 5:49 hours of sunlight on Dec. 21, 2014
3. Oslo, Norway, had about 5:54 hours of sunlight on Dec. 21, 2014
4. Tallinn, Estonia, had about 6:03 hours of sunlight on Dec. 21, 2014
5. Stockholm, Sweden, had about 6:05 hours of sunlight on Dec. 21, 2014
By comparison, New York City's shortest day is about 9:15 hours of daylight and June 21, the longest day, is about 15:05 hours.
Miami, closer to the equator, receives 10:32 hours of sunlight on Dec. 21.
Meanwhile, Reykjavik's longest day in the summer, on June 21, is 21 hours and 45 minutes long.
With a latitude of about 64 degrees north, Reykjavik is the northernmost capital on our planet, with the shortest hours of daylight during the winter and the longest hours in the summer.
On Dec. 21 the sun barely rises at 11:22 a.m.; it rests low in the horizon radiating a soft, low light until it vanishes below the horizon by 3:29 p.m., according to timeanddate.com.
Still, residents in Reykjavik must adapt and continue their lifestyle during the dark winter.
"Like every normal day, we go to work, the kids to school, go to the library, our thermal pools, mountain hiking, cinema, theatre and concerts," said Drífa Magnúsdóttir, project manager at visitreykjavik.com's Tourist Information Center. "Business is the same, people don't change their daily routine during the darkest month, probably we more change our routine during the brightest days, and then we spend more time outside."
Festivities celebrating light have become an integral part of Iceland's winter culture, helping the people persevere through the dark, cold days. In early February, Reykjavik holds a Winter Light Festival, Magnúsdóttir said.
"The darkest days of the year are in December, when we light up the city with Christmas lights," Magnúsdóttir said.
Still, even tourists are drawn to Iceland during the winter to see sites that can only be seen amid darkness.
Magnúsdóttir said the northern lights, a natural light display most clearly seen against a dark sky, are visible only during winter.
This capital is ranked fifth for latitude, located at about 59 degrees north. The long hours of darkness and cold are the reference for another holiday where people reclaim light in its absence.
St. Lucia Day or St. Lucy Day, celebrated most widely across Sweden, but in other areas of Europe as well, is celebrated on Dec. 13, the Winter Solstice in the old Julian calendar.
In Sweden, many believe that Lucia refers to "Lux," meaning light.
The holiday is celebrated at home, in churches, schools and entire towns, where both boys and girls wear white robes while singing Swedish songs; the girls carry candles and the boys carry stars.
One girl is chosen as the Lucia; she wears a crown of candles on her head.
Psychological Effects of Dark Winters
The length of the day has a strong impact on an individual's mood. Seasonal Affective Disorder is a depression related to the season, which arises most often during the winter.
The photoperiod, which is the length of the day from dawn to dusk, is the strongest predictor of mood in patients with Seasonal Affective Disorder, said Kelly Rohan, professor and director of clinical training in the department of Psychological Science at the University of Vermont.
The length of the day is a stronger predictor than weather variables such as temperature, wind speed, brightness of sunlight and precipitation, Rohan said.
"SAD patients feel worse on the short days in the winter and best on long days in the summer," she said.
When the sun rises later in the winter, peoples biological clocks run slow causing a "phase delay" in circadian rhythms, Rohan said. Longer nights might extend the time when the brain releases melatonin, a hormone that helps keep you asleep.