Acute lack of sunlight exposure is associated with poorer cognitive test performance: evidence from two national cohorts in the United Kingdom.

Insufficient sunlight exposure is a growing public health concern in low-sunlight countries, yet its acute effects on cognitive performance remain poorly understood. We examined associations between short-term sunlight exposure and cognitive test performance in two UK national cohorts. Data were obtained from participants of the UK Household Longitudinal Study (UKHLS) and the 1958 National Child Development Study (NCDS), linked to daily solar radiation derived from ground-based monitoring stations. Cognitive outcomes covered memory, verbal, and numerical domains. We exploited cognitive test dates as a natural experiment and employed a time-stratified design, leveraging the assumption that, within strata of the same day-of-week, month, and year, test scheduling was unrelated to both participants' time-invariant characteristics and prevailing weather conditions. This assumption was empirically checked and verified. Time stratification also accounts for seasonality, slowly varying confounders, and long-term trends. Fixed-effects models with restricted cubic splines were fitted to examine nonlinear associations between two-day average solar radiation (on the day of and day preceding the test) and cognitive test scores, adjusting for temperature and rainfall. Positive nonlinear associations were observed between two-day average solar radiation and general cognitive ability, immediate and delayed word recall, and verbal fluency in the UKHLS, but not numerical ability. Similar patterns were found for reading comprehension and mathematics in the NCDS. Associations were strongest at lower baseline levels of solar radiation (levels typical of UK winter and autumn), attenuating or becoming null beyond higher exposure thresholds. Findings were robust to additional adjustment for socioeconomic and demographic characteristics, and similar patterns were observed using an independent satellite-derived ultraviolet radiation measure. Findings suggest that insufficient sunlight exposure is a risk factor for poorer cognitive performance. Increases in solar radiation equivalent to a sunny winter day may be sufficient to confer cognitive performance benefits in settings where solar radiation levels are relatively low, implying that relatively low, safe doses of sunlight could be meaningful.