The mechanisms underlying circadian misalignment, sleep restriction and metabolic disease are not fully understood. We have been investigating the potential of metabolic profiling (metabolomics), the large-scale study of metabolites, to uncover novel mechanisms and possible biomarkers of circadian disruption and sleep deprivation. Using controlled laboratory studies, the effect of sleep and total sleep deprivation on the human metabolome has been characterised. Moreover, whether 24-h metabolite rhythms in plasma are driven by exogenous, imposed factors such as feeding/fasting and sleep/wake and/or by the endogenous circadian timing system has been investigated. Characterising the effects of sleep and food timing on metabolite rhythms in healthy volunteers on normal sleep schedules using targeted LC-MS/MS metabolomics has provided the necessary baseline to our subsequent studies of the metabolomics of shift work.

Two approaches to perform metabolic profiling in shift work have been employed: 1. Simulated shift work protocols in controlled laboratory conditions and 2. Real-life workers doing rotating shifts. Simulated shift work allows light/dark, feeding/fasting and sleep/wake timings to be precisely controlled and multiple time-series samples to be collected under constant routine conditions. Our simulated shift work study showed that after 3 nights of working shifts, endogenous circadian rhythms of many plasma metabolites were misaligned from the central circadian clock timing by ~12 h (internal desynchrony) and instead aligned with the food and sleep timing of the prior shift schedule, likely reflecting the peripheral clocks’ response to mistimed behavioural cues.

Since there is a limit to the number of sequential blood samples that can be collected in field studies, we have recently tested an ambulatory microdialysis device, U-RHYTHM, capable of sampling human interstitial fluid metabolites every 20 minutes for up to 27 h. Targeted metabolomics analysis revealed interstitial fluid metabolite rhythms that correlated with the plasma metabolite rhythms, validating this approach.

In conclusion, misalignment between circulating metabolite rhythms and central circadian clock-driven rhythms (melatonin and cortisol) likely underlies the adverse metabolic consequences of working shifts. Metabolic profiling will be useful to track circadian misalignment in shift work and test management strategies. Combined with U-RHYTHM, a practical way of examining circadian/ultradian metabolite rhythms in real-life shift workers is now possible.