How many parents would leave their babies to sleep outside in the cold? The answer will be none, right? Well for those that live outside Denmark, Finland, Norway and Sweden, the answer may very well be no way, but for most parents in those Nordic countries, the answer could not be a clearer yes to sub-zero temperatures sleep for babies.
In the winter months temperatures in the Nordic countries can be considerably low. Using Stockholm in Sweden as an example average, the lowest winter temperature is typically around -4°C (24F) . Babies in these countries happily sleep outdoors in their prams outside cafés, on porches, in gardens and on balconies. Most impressively of all are the adapted barns/rooms at nursery, designed to allow the flow of cold air for babies and kids while they sleep. This practice has existed for decades.
What is Sub-Zero Temperature Sleep?
Sub-zero temperature sleep is when parents or care-givers put their babies outdoors to sleep. The theory underlying this Nordic custom is that babies, who are exposed to fresh air, are less likely to be affected by colds and other illnesses.
The practice started in the 1940s when child mortality was high and air quality in the home was poor. It was thought that the sunshine and fresh air would prevent rickets and increase immunity against bacteria .
Supporting this notion is anecdotal evidence from parents who believe that their babies sleep better and longer when they are outside. So much so, that it is very commonplace for parents to leave babies in prams or strollers outside cafés in the snow and the cold while they go in for a coffee. This is a cultural norm.
What do we know about sleep?
So, we spend about eight hours a day, 56 hours a week, 240 hours a month and 2,920 hours a year sleeping. Sleep is a pivotal process that occupies almost one third of humans' lives . It is important for physical health, learning and memory, and emotional stability .
Whilst it may appear that nothing is happening inside your head, your brain is very active. With the use of a machine called an electroencephalograph, an encephalogram (EEG) showing brain activity (the wavy lines on the EEG are known as "brain waves") is produced giving insights into what happens when we sleep.
There are two basic forms of sleep: Rapid Eye Movement (REM) sleep and Non-Rapid Eye Movement (NREM) sleep [5–6]. Each form is characterised by a different type of brain wave activity. NREM sleep is the first step, followed by a shorter period of REM sleep, and then the cycle starts over again. There are three stages of NREM sleep and each stage can last from five to 15 minutes. You go through all three stages before reaching REM sleep:
In REM sleep your eyes move quickly in different directions, dreams normally occur during this time, sometimes you can have intense dreams during REM sleep, since your brain is more active. Normally, REM sleep happens 90 minutes after you fall asleep: the first period lasts for around ten minutes, each of your later REM stages gets longer, and the final one may last up to an hour. Your heart rate and breathing increase. Waking up after a full night's rest is normally from REM sleep.
Overall babies spend about 50% of their sleep time in REM sleep and 50% in NREM. Adults spend about 20% of their sleep time in REM and 80% in NREM sleep. Elderly people spend less than 15% of their sleep time in REM sleep.
Are there any benefits of cold weather sleeping for babies?
Quantifying the health benefits of sub-zero temperature sleep for babies is difficult: research in this area is in its early infancy. For the general population there are some surprising health benefits to cold weather; such as calorie burning, bringing loved ones close to each other for warmth, stamping out disease-carrying bugs or viruses, appreciation of good weather, potential reduction with inflammation, and an escape from the pressures to get a bikini body . From what is known, babies have the following benefits:
What in general are the potential issues with cold weather exposure?
Prolonged exposure to severe cold is not advisable, adverse health effects from exposure to the cold varies from person to person and the conditions can also vary [8–9]. Aside from the normal colds, flus, sore throats, potential conditions resulting from sub-zero temperature naps for babies can include:
Why then would I be interested in putting my baby outdoors in the cold to sleep?
Considering babies sleep for approximately 14 months out of their first 24 months of life , any help a parent can get to develop their growth is a positive thing. Children's sleep durations are steadily declining , and paediatric sleep problems are persistent [15–23] despite the critical role that sleep plays in children's development and daily functioning. 20 to 30% of children experience clinically significant sleep problems , and approximately 70% of children experience at least one sleep-related problem a few nights a week .
Parents who undertake the practice of sub-zero temperature sleep report their babies sleeping for longer in the fresh air. Overall there are many factors, such as health states, emotional states, bedding conditions and thermal environments which affect sleep quality. Thermal environment has been found to be the most important factor [25–26]. Okamoto-Mizuno and his team  found;
This finding explains what the Nordic parents and care providers report when they say their babies sleep for longer and feel refreshed in the cold.
Is there any evidence to support Sub-Zero Temperature Sleep apart from general sleep studies?
A study conducted in Finland by Tourula and team , looked at parents’ opinions about their children sleeping outdoors during a Finnish winter. Overall the researchers said that parents expressed subjective and mainly positive experiences and that most had not faced potentially dangerous situations.
Research being conducted by Prosser Hannu Rintamaki at the Finnish Institute of Occupational Health also supports the evidence that babies do sleep for longer . Prof Hannu Rintamaki had this to say on the matter:
Babies sleep much better in the cold than in the warm environment. The sleeping time is 2.3 times longer usually in the cold. But the thermal insulations should be optimal, the baby should not be overheated or either cooled down.
Further results from Prosser Hannu Rintamaki’s research showed that parents felt their children were more active and also ate better after sleeping outside. With over 94% of those surveyed saying that the fresh air was good for their child’s health .
Are there any anecdotal or studies against Sub-Zero Temperatures Sleeping for babies?
There is no real evidence against the practice of sub-zero temperature sleep for babies, with most studies showing little-to-no difference. However, Margareta Blennow, a paediatrician in Sweden with an interest in this topic, says that reports gathered from Sweden's Environmental Protection Agency showed that some pre-schoolers who spent a lot of time outdoors were more likely to miss a few days of school because of sickness:
Some studies show that pre-schoolers who spent several hours in the open, not just sleeping, miss[ing] fewer days of kindergarten than those who spend more time indoors. But other studies have found a difference.
Svante Norgren, yet another paediatrician who also happens to be the director at the Astrid Lindgren Children’s Hospital in Stockholm, Sweden, said precautions must be taken by making sure that children are dressed appropriately. He also offered advice to parents for babies less than two weeks old, that below zero the babies should not be placed outdoors to sleep:
We have had new-borns admitted to the ER with hypothermia.
How to keep babies warm when they sleep in Sub-Zero Temperatures
The benefits of sub-zero napping begin to outweigh the preliminary fears of putting your child outside to sleep, and there is some advice on how you can keep your babies safe and warm , after all, according to those in Nordic countries there is no bad weather, only bad clothing.
All sensible parents and carers in Scandinavia engaging in sub-zero temperature baby sleep bring the children indoors when the weather drops to -10°C. But there are the hardened minority that will keep children out to -20°C, as indicated by the head teacher at a Stockholm centre, Brittmarie Carlzon, when interviewed by the BBC :
When the temperature drops to -15°C (5F) we always cover the prams with blankets.
More advice was offered by Jarnstrom Martin, who is head of a group of nurseries in Sweden. She is a massive supporter of sleeping outdoors, however she stresses the importance of keeping the baby warm when the weather is cold.
It is very important for babies to have wool close to the body, warm clothes and a warm sleeping bag.
Patricia Clayhills-Siljama, a Health Visitor at the Helsinki Public Health Services offers some advice on how to integrate babies to the practice of sub-zero temperature sleep :
If it is a winter baby it might be a good idea to take the baby out for just 10, 15, 30 minutes to start with. And of course you also have to use [the] common sense and see what kind of weather it is and some kind of line is -10°C, so over that it might not be such a good idea to take the babies out for longer
While the scientific evidence is not conclusive as to the benefits of the sub-zero temperature sleep for babies, there is also no real evidence against it, with researchers and health professionals advising parents to apply common sense. Provided the advice on keeping babies warm while they sleep outside is followed, then the correct thermal environment can be created, allowing the baby sleep peacefully!