Daylight matters

A blog by Timo Partonen

From November to January, if your exposure to daylight is of 1 hour or longer per day, it is likely that you will not have depressive symptoms during that period of the year. Compared to the conditions where your exposure of less than 1 hour per day, the odds of having depressive symptoms will be 0.72 on average, yielding the confidence interval from 0.60 to 0.82. If you have children younger than 12 years of age at home, it is even more likely that you will not have depressive symptoms from November to January, if your exposure to daylight is of 1 hour or longer per day. These findings, of the total of 23,581 observations, were derived from 10,430 persons living in Sweden (Raza et al. 2024).

Self-reports were corroborated by the all-wavelength readouts of solar radiation, with a horizontal resolution of 11 km × 11 km and a temporal resolution of 1 hour. The individual readouts demonstrated that the greater the exposure to solar radiation in residential neighborhood was, the less frequent the depressive symptoms were. I think that this is good news. Because, first, I see that they are generalizable to people living in high latitude regions, and second, they open a way to adapt to climate change.

Photo by brooklyn on Unsplash

In high latitude regions, the seasonal variation in natural light is an essential characteristic of the climate. As climate change is already quickly warming up the Arctic and subarctic regions, there will be significant changes in solar radiation as well. Solar radiation is projected, e.g., in northern Finland by the end of this century, to increase by about 5% in summertime and to decrease by 10% to 20% in wintertime. From the perspective of public health, the outcome is likely to turn out negative, if left unnoticed.

On the one hand, less solar radiation in wintertime will turn the morning hours darker, which tends to result in greater than usual delays of the circadian pacemaker. It is thereafter likely that sleep disturbance as well as depressive symptoms together with carbohydrate craving in the evening hours will emerge more frequently than as usual. On the other hand, more solar radiation in summertime will turn the evening hours lighter, which leads to greater than usual delays of the circadian pacemaker as well. As a result, more frequent sleep disturbance is likely, such as among 1962 individuals of a cohort of the general population being exposed to greater solar radiation in their residential neighborhood during the past year (Elovainio et al. 2022).

Albeit winter depression has been studied for decades and associated with low exposure to daylight, the influence of global radiation on depression remains rather unexplored. There are some reports though. A study from the Netherlands (Sarran et al. 2017), with the total of 23,197 assessments of depressive symptoms weekly during the winter months over six years, included 291 patients having winter depression and living in and near Groningen. It suggested that forecasting sunshine duration or cloud cover might improve the possibilities of early intervention. Another study from Finland (Komulainen et al. 2022) demonstrated that 1845 individuals of a population-based cohort being exposed to greater solar radiation in their residential neighborhood during the past year were less likely to report suicidal thoughts.

To adapt to slowly emerging impacts of climate change, we need to rethink, (a) how to design new and renovate old buildings to let daylight come in from outside, (b) how to time artificial light exposures during the day to align with the circadian pacemaker, and (c) how to spend our time in daylight. Future is now, is not it.

Further reading

Elovainio M, Komulainen K, Lipsanen J, Partonen T, Pesonen AK, Pulkki-Råback L, Paunio T, Kähönen M, Vahtera J, Virtanen M, Ruuhela R, Hakulinen C, Raitakari O. Long-term cumulative light exposure from the natural environment and sleep: a cohort study. J Sleep Res 2022; 31: e13511. https://doi.org/10.1111/jsr.13511

Komulainen K, Hakulinen C, Lipsanen J, Partonen T, Pulkki-Råback L, Kähönen M, Virtanen M, Ruuhela R, Raitakari O, Elovainio M. Associations of long-term solar insolation with specific depressive symptoms: evidence from a prospective cohort study. J Psychiatr Res 2022; 151: 606–610. https://doi.org/10.1016/j.jpsychires.2022.05.038

Raza A, Partonen T, Hanson LM, Asp M, Engström E, Westerlund H, Halonen JI. Daylight during winters and symptoms of depression and sleep problems: a within-individual analysis. Environ Int 2024; 183: 108413. https://doi.org/10.1016/j.envint.2023.108413

Sarran C, Albers C, Sachon P, Meesters Y. Meteorological analysis of symptom data for people with seasonal affective disorder. Psychiatry Res 2017; 257: 501–505. https://doi.org/10.1016/j.psychres.2017.08.019