As the days become shorter and nights get longer, many brace for the winter blues. But what actually causes them?  

For decades, seasonal depression has been blamed on the lack of sunlight, but a new international study suggests that the real problem may not be darkness itself—but instead the artificial lights often used to brighten the long dark winter nights. 

Researchers found that exposure to “artificial light at night”, or “ALAN” for short, may significantly increase the risk of depression in older adults.  

The study analyzed data from nearly 40,000 people aged 50 and above across the U.S., England, Northern Ireland and Ireland, and used satellite data to measure nighttime lights levels in the participants’ areas compared to their mental health scores. 

The results indicated that that those living in the areas with more artificial lights were more likely to report symptoms of depression that those who lived in darker areas. 

In Northern Ireland, people in the most light-polluted areas were nearly 80 percent more likely to experience depression. The results held up across all four countries, albeit to different degrees, even after adjusting for other factors like air pollution, income and neighbourhood conditions. 

This study also examined how the effects varied between urban and rural areas. In England and Ireland, rural residents appeared more vulnerable to the effects of ALAN, while in the U.S. it was the opposite. 

Experts believe that ALAN can cause circadian disruption, messing with our natural rhythms and internal clock. This can interfere with sleep quality, hormone production and emotional regulation. 

While this study is one of the most comprehensive on the subject, scientists note that more precise methods are needed to fully understand all the ways in which exposure to artificial lighting can affect mental health. 

Do you have a tip on a health story that Newsweek should be covering? Do you have a question about ALAN? Let us know via health@newsweek.com.

Reference

So, R., D’Souza, J., Feeney, J., Küçükali, H., Keller, K. P., Di Gessa, G., Valson, J. S., Hunter, R. F., McGuinness, B., Kee, F., Nolan, A., Mc Loughlin, S., Lee, J., Adar, S. D., & Zaninotto, P. (2025). Artificial outdoor light at night and depression in older adults in the USA, England, Northern Ireland, and Ireland. Environment International, 205. https://doi.org/10.1016/j.envint.2025.109886