Sunlight Exposure and Suicide Risk – A conversation with environmental health researcher Shinsuke Tanaka

1. What initially prompted you to question how sunlight had been studied in relation to suicide risk?

Much of the earlier literature suggested a positive association between sunlight and suicide, which you describe as counterintuitive. What made you suspect that the issue might lie in how sunlight was being measured rather than in sunlight itself?

Shinsuke Tanaka: What struck me was the mismatch between the common clinical intuition – insufficient winter sunlight is linked to depressive symptoms like Seasonal Affective Disorder – and the fact that suicide tends to peak in late spring/early summer. That tension made me look closely at prior designs and realise that many “positive” sunlight–suicide findings can arise mechanically because both sunlight and suicide share strong seasonal patterns, creating a spurious correlation when seasonality isn’t handled granularly enough.

2. Your study uses satellite-based measurements of solar radiation rather than daylight hours.

Why is this distinction so important? For readers without a technical background, could you explain why measuring actual solar radiation reaching the Earth’s surface provides a more meaningful picture of sunlight exposure?

Shinsuke Tanaka: Daylight hours tell you how long the sun is above the horizon, but not how much usable light actually reaches people at the surface. Solar insolation directly measures the energy hitting the Earth’s surface and varies with cloud cover, atmospheric conditions, sun angle, and related factors, so two places can have the same day length but very different real exposure. In practical terms, our satellite-based measure is designed to capture the intensity of sunlight at the surface (not just sunrise-to-sunset time), which is closer to the biologically relevant exposure. 

3. Your findings show a negative association between sunlight exposure and suicide rates. How robust is this relationship?

Could you briefly describe how your analysis accounts for seasonality, geographic differences, and other potential confounding factors?

Shinsuke Tanaka: The relationship is highly robust. We used a method that compares the exact same location and month across different years, for instance, comparing a particularly sunny August in one county to a cloudy August in the same county a year later. This allows us to filter out regular seasonal patterns, like school calendars or holidays. By adjusting for yearly trends within each state or even county, we also ensured that broader economic shifts, such as changes in unemployment or poverty rates, did not influence our results. Finally, by also accounting for temperature and rainfall, we can confirm that the effect is driven specifically by sunlight intensity rather than other environmental factors.

4. You report that a one-standard-deviation decrease in sunlight exposure is associated with a 6.76% increase in suicide rates.

How should we understand the magnitude of this effect? How does this compare with other known suicide risk factors or public health interventions?

Shinsuke Tanaka: A one-standard-deviation drop in sunlight is roughly comparable to the difference between lowest- and highest-sunlight state averages (i.e., Vermont vs. Arizona). In terms of scale, the paper shows this effect is comparable to, and sometimes larger than, effect sizes reported for other factors/interventions studied in the suicide literature (e.g., temperature, air pollution, and major policy interventions), underscoring sunlight as a quantitatively meaningful risk factor.

5. Suicide rates are often highest in spring rather than winter.

How does your work help explain this apparent paradox? Does your research suggest that changes in sunlight exposure may be as important as absolute levels?

Shinsuke Tanaka: First, the “spring peak” is part of the seasonal average pattern, but our design removes that baseline seasonality at the county-by-month level and instead uses unusual deviations in sunlight for a given place and month to estimate the effect.

Second, we find sunlight matters not only contemporaneously but also with a meaningful lag (especially the previous month), which means low sunlight in late winter can plausibly carry into elevated spring risk, even as overall daylight is increasing.

6. Your paper also raises concerns about solar geoengineering.

Why was it important to include this perspective? What kinds of unintended mental-health consequences should policymakers consider when evaluating technologies designed to reduce sunlight exposure?

Shinsuke Tanaka: Solar geoengineering proposals aim to cool the planet by reflecting sunlight back into space, potentially deployable relatively quickly, but with substantial uncertainty about broader human and environmental impacts. Because our results indicate that reduced sunlight can worsen mental well-being and increase suicide risk, a technology that intentionally reduces surface sunlight could carry unintended mental-health harms that should be evaluated alongside temperature benefits.

Interview by: Ania D., Sun for Life
Published with permission