Investigating the effects of circadian rhythm on the human skin lipidome.

The circadian rhythm is a 24 h cycle that harmonises the activity of organs - including the skin - to a daily routine using neurological and hormonal signals. Limited research has been done to understand the effects of the circadian rhythm on the skin lipidome. We used reversed-phase liquid chromatography-mass spectrometry (RPLC-MS) in a longitudinal study to investigate temporal changes to the skin lipidome over a 24 h cycle for eight healthy participants. All statistical analyses were performed with a group-mean and individual-mean data approach. Using cosinor analysis -values, a total of 29 metabolites (0.67% of all detected metabolites) exhibited a statistically significant circadian rhythmicity across participants; however, individually, a range of 3.51-18.53% of metabolites were considered rhythmic. The use of FDR -values and Lomb-Scargle analysis showed no circadian metabolites. Using PCA and PLS-DA, no significant clustering based on timepoints was observed across participants; however, half of individuals showed significant metabolite clustering at 07:30. Further, sebum-specific squalene and sapienic acid as well as stratum corneum-specific cholesterol sulfate showed no significant differences in concentrations across timepoints. While individuals exhibited temporal differences, as an averaged healthy cohort the impacts by the circadian rhythm or time of sampling were considered negligible.