Would you put your baby or toddler outside in the freezing cold for their lunchtime nap? Most Nordic parents wouldn't give it a second thought. For them it's part of their daily routine.

Daytime temperatures this winter in Stockholm have regularly dropped to -5C (23F) but it's still common to see children left outside by their parents for a sleep in the pram.

Wander through the snowy city and you'll see buggies lined up outside coffee shops while parents sip on lattes inside.

And if you are visiting friends and your child needs a nap, you may be offered the garden or balcony instead of a bedroom.

"I think it's good for them to be in the fresh air as soon as possible," says Lisa Mardon, a mother-of-three from Stockholm, who works for a food distribution company.

When the temperature drops to -15C we always cover the prams with blankets Brittmarie Carlzon, Pre-school head teacher

"Especially in the winter when there's lots of diseases going around... the kids seem healthier."

Her children have been sleeping outside since they were born.

The youngest, Alfred, is two and she puts him outside in the pram to nap once a day, for an hour and a half. When he was younger he slept outside twice a day.

This isn't a recent fashion. Lisa's mother, Gunilla, now 61, says she also did it with Lisa when she was a baby.

"Yes we were doing it back then as well… It was important for her to get fresh air and stay healthy," Gunilla says.

And Lisa's father, Peter, was put outside by his mother to sleep in a pram in the 1950s. Only when it got to around -10C (14F) did she bring him indoors.

Nowadays most day-care centres in Sweden put children outside to rest. It's common to see rows of prams lined up in the snow at nap-time, with youngsters fast asleep inside.

Image caption The thermometer shows an outside temperature of -10C

At Forskolan Orren, a pre-school outside Stockholm, all children sleep outside until they reach the age of three.

"When the temperature drops to -15C (5F) we always cover the prams with blankets," says head teacher Brittmarie Carlzon.

"It's not only the temperature that matters, it's also how cold it feels. Some days it can be -15C but it actually feels like -20C (-4F) because of the wind," she says.

Image caption Babies often sleep outside cafes in Copenhagen too

"Last year we had a couple of days with a temperature of -20C. On those days we brought the prams inside some of the time the children were sleeping, but most of their sleep they spent outdoors."

One group at the pre-school spends all its time outside, from 09:00 to 15:00 every day. Out in the fresh air they do everything children normally do inside, only going inside at mealtimes, or in unusually cold weather.

The theory behind outdoor napping is that children exposed to fresh air, whether in summer or the depths of winter, are less likely to catch coughs and colds - and that spending a whole day in one room with 30 other children does them no good at all.

Many parents also believe their children sleep better and for longer in the open, and one researcher in Finland - outdoor napping is popular in all the Nordic countries - says she has evidence from a survey of parents to back this up.

"Babies clearly slept longer outdoors than indoors," says Marjo Tourula. While indoor naps lasted between one and two hours, outdoor naps lasted from 1.5 to three hours.

"Probably the restriction of movements by clothing could increase the length of sleep, and a cold environment makes swaddling possible without overheating," she says.

Finland's 'official' nap advice Irrespective of the season, many children have their evening naps outside in prams. Many babies sleep better outdoors in the fresh air than in the bedroom. Sleeping outdoors is not dangerous for a baby. One may gradually start going outdoors when the baby is two weeks old. Read more: Having children in Finland

According to her research, -5C is the best temperature for an outdoor nap - though some parents she spoke to even put their children out at -30C.

But do children who sleep outside end up catching fewer coughs and colds?

Paediatrician Margareta Blennow says reports from the Swedish Environmental protection agency show conflicting results.

"In some studies they found pre-schoolers who spent many hours outside generally - not just for naps - took fewer days off than those who spent most of their time indoors," she says.

"In other studies there wasn't a difference."

Martin Jarnstrom, head of one of the Ur och Skur group of pre-schools , is another big advocate of outdoor naps, though he emphasises that while the weather may be cold, the child must be warm.

"It's very important that the children have wool closest to their body, warm clothes and a warm sleeping bag," he says.

There is a Swedish saying that encapsulates this thought - "There is no bad weather, only bad clothing."

Another saying sums up what Swedes are likely to think when toddlers in other countries are kept indoors in sub-zero temperatures: "A little fresh air never hurt anyone."

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