Washout and photostability of hair endocannabinoids and N-acylethanolamines: Segmental stability, head-covering effects, and in vitro sunlight exposure.

Hair analysis has increasingly emerged as an innovative method capable of effectively tracking the cumulative secretion of endogenous cannabinoids (ECs) and N-acylethanolamines (NAEs) over retrospective periods. However, a potentially influential covariate that remains underexamined is hair-shaft exposure to natural sunlight. The present study comprises three experimental investigations to systematically examine the photostability of ECs and NAEs in hair exposed to natural sunlight. Experiment 1 assessed segmental profiles in 6-cm strands cut into six 1-cm segments, testing stability across segments in a repeated-measures design. Experiment 2 examined head-covering and segment effects on 18-cm strands (six 3-cm segments) from individuals who wore a hijab daily vs. those who did not. Experiment 3 exposed matched hair samples in vitro to natural sunlight 0, 1, or 6 h/day for one month. The study shows that the washout effect along the hair shaft with 1-cm segment was observed for hair AEA but no significant pairwise differences after correction, while other analyte concentrations showed no overall segmental change. In the head-covering study, segmental effects were significant for most hair analytes, except for hair AEA in the non-hijab group. No significant interaction effect between head-covering and segmental profile was observed for any hair analytes between groups except for hair SEA. Sunlight exposure produced a robust, dose-responsive reduction in hair AEA, while the patterns for 1-AG/2-AG and NAEs were smaller or inconsistent across exposure levels. Natural sunlight is a consequential covariate for hair-based EC/NAE biomarkers, most notably reducing AEA under natural sunlight exposure. Segmental proximal to distal gradients and head-covering practices can modulate levels. These findings provide an important experimental and methodological foundation for the application of ECs and NAEs in hair as reliable biomarkers, strengthening future research on long-term physiological monitoring in psychoneuroendocrinology.