The Impact of Winter Urban Light on Night-Time Sleep

Background

Light and darkness are the primary environmental factors regulating human circadian rhythms. In natural environments, humans are exposed to light levels ranging from 3,000 lux (cloudy winter sky) to 100,000 lux (clear sky). However, in modern urban environments, especially during winter, people are often exposed to extremely low light levels, a phenomenon referred to as “biological darkness.” This low-light environment may have profound effects on human sleep architecture and mental health. Previous studies have shown that insufficient light exposure is associated with biological markers of mental illnesses such as depression, including shortened REM sleep latency. However, research on the specific effects of low light on sleep architecture in healthy populations remains limited.

This study aims to explore the impact of daytime light in urban winter environments on objective night-time sleep, particularly on REM sleep and slow-wave sleep (SWS). The research team hopes to quantify daytime light levels in urban winter and reveal their relationship with night-time sleep parameters, thereby providing new insights into the effects of light on human circadian rhythms.

Source of the Paper

This paper was co-authored by Claudia Nowozin, Amely Wahnschaffe, Jan de Zeeuw, and others from the Institute of Physiology at Charité – Universitätsmedizin Berlin and the Clinic Sleep & Chronomedicine in Germany. The paper was published in 2025 in the European Journal of Neuroscience under the title Living in Biological Darkness II: Impact of Winter Habitual Daytime Light on Night-Time Sleep.

Research Process

Study Participants and Design

The study recruited 11 healthy participants (mean age 25.4 years, 6 males, 5 females) who wore eyeglass frames equipped with light sensors for four consecutive days during the winter of 2008, recording daytime light intensity (range: 1-40,000 lux). Among them, nine participants underwent two nights of polysomnography (PSG) monitoring in the laboratory to assess night-time sleep architecture.

Light Measurement

Light sensors were vertically mounted on the eyeglass frames, with a recording frequency of 1 Hz. The research team cleaned the raw data, excluding anomalies below 1 lux and above 40,000 lux, ultimately using 87.2% of the valid data. Light data were divided into time periods: morning (7:00-11:00), midday (11:00-15:00), and afternoon (15:00-19:00), with median light intensity calculated for each period.

Sleep Monitoring

Participants spent one night in the laboratory under low-light conditions ( lux) and underwent PSG monitoring. PSG data were scored according to the American Academy of Sleep Medicine (AASM) standards, recording REM sleep latency, REM sleep density, REM polarity (distribution of REM at the end of sleep), and the night-time distribution of slow-wave sleep (SWS).

Key Findings

Daytime Light Levels

The study found that daytime light levels in urban winter environments were extremely low. The median light intensity for the entire day (7:00-19:00) was only 23 lux, with 81 lux in the morning, 68 lux at midday, and 22 lux in the afternoon. Participants were exposed to light levels above 500 lux for only 36 minutes per day.

Relationship Between Light and Sleep Parameters

• REM Sleep Latency: Light intensity at midday (11:00-13:00) was significantly correlated with REM sleep latency (rho = 0.817, p = 0.049). Lower midday light levels were associated with shorter REM sleep latency.
• REM Polarity: Lower midday light intensity was associated with REM polarity shifting toward the early stages of sleep (rho = 0.817, p = 0.049).
• Non-REM Sleep: Light intensity in the afternoon (15:00-17:00) was negatively correlated with the duration of N2 sleep stage (rho = -0.883, p = 0.014).

Discussion and Conclusion

The results indicate that low light levels in urban winter environments significantly affect night-time sleep architecture, particularly the distribution and latency of REM sleep. These changes resemble sleep patterns observed in patients with depression, suggesting that low-light environments may have potential implications for mental health. The study also proposes that the human physiological system appears to adapt to this low-light environment and responds to variations in light intensity under such conditions.

Highlights of the Study

1. Quantification of Low-Light Environments: The study is the first to quantify daytime light levels in urban winter, revealing the phenomenon of “biological darkness.”
2. Relationship Between Light and Sleep: The study found that even at extremely low light levels, variations in light intensity can significantly affect night-time sleep architecture.
3. Similarity to Depression Sleep Patterns: The results resemble sleep patterns in patients with depression, providing a new perspective on the relationship between light and mental health.

Research Significance

This study not only reveals the impact of insufficient light in urban winter environments on human sleep but also provides important data for future research on the relationship between light and mental health. The findings suggest that optimizing daytime light conditions may help improve sleep quality and mental health, particularly in winter and urban environments. Additionally, the concept of “biological darkness” introduced in the study offers a new theoretical framework for understanding the effects of modern lifestyles on human circadian rhythms.

Other Valuable Information

The research team also developed a new mathematical tool to quantify the distribution (polarity) of REM and SWS during night-time sleep. This method represents each 30-second sleep stage as a vector and sums them in a 24-hour polar coordinate system to calculate the “polarity” time and strength of REM and SWS. This approach provides a new analytical tool for sleep research.

By quantifying light levels in urban winter environments and their impact on sleep, this study offers important insights into the relationship between light and human circadian rhythms, laying the groundwork for future mental health research.