Sunshine increases placebo effect in major depressive disorder (MDD) patients (Gailledreau et al., 2015). Kokras et al. (2014) showed that sunshine induces different responses in female than male mice in preclinical models of depression.

To determine whetehr the sunshine induced placebo effect exhibits gender differences in human.

Data from 9 double-blind, randomized, placebo-controlled studies of antidepressants conducted by the French GICIPI network were reviewed. MADRS (5) or HAM-D 17 (4) were used as the main efficacy tool. For each patient, variation of scores (Delta MADRS/Delta HAM-D) between two consecutive visits were correlated with the average sunshine index observed at noon between these visits. Sunshine indexes were provided by Météo-France. Correlations were computed with Microsoft Excel.

Analysis of both genders (n = 52) showed no statistically significant (NS) correlation (r2 = 0.0064) between sunshine and score variations. Analysis of males (n = 8) failed to demonstrate any significant correlation in cloudy (< 1000 Joules/cm2), variable (1000–2000 Joules/cm2) or sunny (> 2000 Joules/cm2) weather. Analysis of females (n = 44) showed NS correlation as well for cloudy or variable weather (r2 = 0.0016), but a strong correlation was observed for females exposed to sunny weather: r2 = 0, 315, n = 20, P < 0.01. This correlation was even stronger in the subpopulation of females aged less than 50 years: r2 = 0.6398, n = 12, P < 0.001.

The hypothesis underlying this correlation between sunshine index and variations of MADRS/HAMD scales will be discussed.

Sunshine increases placebo effect in female patients aged less than 50. This insufficiently known effect may be responsible for failure of a number of double-blind, randomized, studies of antidepressant compounds.

The authors have not supplied their declaration of competing interest.