Problem:

I found this article on estimating body time using molecular timetable of 168 genes. researchers looked at gene expressions and plotted them to 24 hour cosines curve with some pretty spectacular results.

Unfortunately the techniques discussed required sampling of liver cells, which may be invasive.

Question: Are there non- invasive methods of estimating body time/ biological clock/ circadian rhythm time?

An example of body time: at 8:00 real time, a mouse may exhibit gene expressions more appropriate for 9:30, so the body time of the mouse is 9:30.