For many organisms, including humans, the activity of certain biological processes and the expression of certain genes and proteins fluctuate predictably over a period of roughly 24 hours. These oscillations are maintained by an internal time-keeping mechanism called the circadian clock. 'Body time' refers to the setting of each individual's circadian clock, which can vary by as much as 12 hours among individuals. Body time can also be influenced by working late-night shifts, suffering from jet lag or lacking regular routines (e.g., sleep, meal times). These individual variations are important because body time affects not only personal comfort and stress levels, but also potency and toxicity of some medications as well as the likelihood of developing diet-induced obesity. Hence, reliable determination of an individual's body time is necessary to facilitate chronotherapy (or matching of drug delivery to body time) and time-restricted diet strategies.

The conventional method for body time determination involves repeated blood sampling for the measurement of cortisol or melatonin over 24 hours or more under controlled environmental conditions. This approach has the advantage of measuring body time directly, but it is time- and labor-intensive and somewhat invasive. As an alternative, a group of Japanese scientists led by Takeya Kasukawa (RIKEN Center for Developmental Biology, Hyogo) and Masahiro Sugimoto (Keio University, Yamagata, and Kyoto University) developed a molecular timetable method for estimating body time. The circadian oscillation patterns of specific genes or metabolites in the body are profiled in detail to create a reference timetable of metabolites whose abundance fluctuates over 24 hours. Scientists can then measure these metabolites in only two samples, taken 12 hours apart, from an individual and compare the measurements to the reference timetable to estimate that individual's body time. Kasukawa and Sugimoto's team previously applied their molecular timetable method to estimating body time in mice using expression of clock-controlled genes and blood plasma metabolites. They now report application of their technique to estimating individual body time in humans.

On the basis of the two anti-phase blood samples, the scientists were able to accurately pinpoint individuals' body times to within 3 hours, even under different experimental conditions (Proc. Natl. Acad. Sci. USA published online 27 August 2012; doi:10.1073/pnas.1207768109). “Our study demonstrates that human internal body time can be detected using limited time-point sampling and a reference metabolite timetable,” wrote the authors. They hope that their technique “may lead to highly optimized and personalized medicine in the future.”