Winter babies: Why do Nordic mothers let their babies sleep outside in Winter?

At the beginning of 2020, a piece of news made the round that mothers in the Nordic countries let their babies sleep outside, even during sub-zero temperatures. We do not want to start any discussion about the safety of leaving a baby unsupervised outside, but rather talk about why it can be beneficial for individuals of any age to sleep with open windows and lower bedroom temperatures.

Studies have already shown that colder bedroom temperatures can improve sleep quality [1]. This can be explained by the natural rhythmicity of the body temperature, which increases in the morning shortly before we wake up and decreases towards the evening to initiate sleepiness [2]. That is why some can sleep better with their feet sticking out of the blanket. It helps the body dump heat via the bottom of the feet. On the opposite, many have experienced how hard it is to fall asleep and get a good night of rest during hot summer days.

Of course, some will point to the fact that we are talking about temperatures below zero degrees Celsius for these babies, a temperature level which will hardly be reached in a bedroom. However, it is important to consider that an air temperature below zero does not necessarily mean that the temperature of the immediate environment of the baby, the air between the blanket and the body, is also below zero degrees Celsius. That brings us to another stream of literature which points to the remarkable adaptability of the human body to cold temperatures [3]. Exposing yourself regularly to cold temperatures improves the body’s metabolic processes in a way that the body can produce more heat to keep itself warm. One major source is brown adipose fat tissue, which functions as firewood. Its sole purpose is to be “burned” to generate heat [4].

Thus, humans are capable of dealing with a slightly colder environment by using fat tissue to generate heat. It is important to mention that this process does not continue endlessly. As with every process of the body, warming up the body requires energy in form of calories which needs to be continuously provided with food. Also, there is a risk that these babies become hypothermic, meaning the body temperature drops below 35 degrees Celsius. This state is lethal if it is lasting because it can lead to complete failure of the heart and respiratory system. Thus, these mothers know that several thick blankets are necessary to keep their babies warm during winter times.

Another factor is air quality. The outside air has lower concentrations of carbon dioxide and air pollutants (assuming you are not sleeping next to a busy road with very high levels of pollutants from traffic). A recent study has shown that good air quality in terms of low levels of carbon dioxide and air pollutants can increase sleep quality [5]. These beneficial effects can be achieved through proper ventilation of the bedroom. That can explain why some people prefer sleeping with open windows throughout the year.

To conclude, if you want your baby to sleep well, or if it is you that is looking for better sleep quality, try to sleep with open windows, in a colder bedroom environment or even outside if convenient. However, keep in mind that if you put your baby outside for sleep, the highest priority should be always the safety of your baby against extreme temperatures (hot and cold) as well as against threats from nature or other individuals.

[1] Lan, L., Tsuzuki, K., Liu, Y. F., & Lian, Z. W. (2017). Thermal environment and sleep quality: A review. Energy and Buildings, 149, 101-113.

[2] Refinetti, R. (2010). The circadian rhythm of body temperature. Frontiers in Bioscience-Landmark, 15(2), 564-594.

[3] Castellani JW, Young AJ. Human physiological responses to cold exposure: Acute responses and acclimatization to prolonged exposure. Auton Neurosci. 2016 Apr;196:63-74. doi: 10.1016/j.autneu.2016.02.009. Epub 2016 Feb 21. PMID: 26924539.

[4] van der Lans, A. A., Hoeks, J., Brans, B., Vijgen, G. H., Visser, M. G., Vosselman, M. J., ... & van Marken Lichtenbelt, W. D. (2013). Cold acclimation recruits human brown fat and increases nonshivering thermogenesis. The Journal of Clinical Investigation, 123(8), 3395-3403.

[5] Liao, C., Akimoto, M., Bivolarova, M. P., Sekhar, C., Laverge, J., Fan, X., ... & Wargocki, P. (2021). A survey of bedroom ventilation types and the subjective sleep quality associated with them in Danish housing. Science of the Total Environment, 798, 149209.