“The circadian rhythm, or circadian clock, is an internal mechanism that drives the 24-hour cycles […] — and now, new research has found that bacteria living within the gut also have a clock.”
Jiffin K. Paulose et al. Human Gut Bacteria Are Sensitive to Melatonin and Express Endogenous Circadian Rhythmicity, PLOS ONE (2016). DOI: 10.1371/journal.pone.0146643