Diurnal rhythm of the human plasma proteome.

Background: Plasma is the most used clinical specimen, yet diurnal variation in plasma proteins remains largely unexplored. We aimed to identify diurnally-regulated proteins in healthy individuals and assess their potential diagnostic implications, and highlight how diurnal awareness can advance future biomarker research.

Methods: Twenty-four healthy young individuals were studied under highly controlled conditions. Venous blood was drawn every three hours over a 24-h period, yielding 216 samples, of which 208 high-quality plasma samples were analyzed via high-throughput mass spectrometry. The missing data were filtered and imputed, and rhythmicity was assessed using Cosinor-based modeling with Benjamini-Hochberg correction. Tissue and pathway enrichment analyses were performed using the DAVID functional annotation tool.

Findings: Of 523 proteins that passed quality thresholds, 138 (~ 26%) exhibited significant diurnal oscillations. Tissue enrichment analysis revealed that most rhythmic proteins originated from the liver and platelets, with additional enrichment in a variety of tissue types. Pathway enrichment showed diurnal regulation of hemostasis, immune signaling, integrin-mediated processes, glucose metabolism, and protein synthesis. Notably, 36 clinically utilized biomarkers, including albumin, amylase, and cystatin C exhibited diurnal variation, suggesting that failing to account for temporal fluctuations may reduce diagnostic precision.

Interpretation: These findings demonstrate that over one-quarter of the human plasma proteome is under diurnal control. Such oscillations might have direct clinical implications, as the time-of-day may alter biomarker accuracy. Incorporating diurnal timing into diagnostic and research protocols, through standardized sampling or time-sensitive reference intervals, could improve patient care and inform future biomarker discoveries. Further research in larger, more diverse populations is needed to generalize these results and streamline practices in a way that takes diurnal variation into account.