Abstract Contrary to what has long been suggested, the biological clock is not a poor timekeeper. In fact, it has been working in precision for millions of years as a genetically set clock, independent of environmental periodicity. However, as the Earth's rotation has gradually slowed, this internal clock has gone through a relative phase shift. Extrapolating of changes backwards through time shows that the clock was set when mammals first appeared on the planet. Interestingly, primates are the only beings that have a free-running cycle longer than 24 hours. This bioclock may be set by lunar entrainment, or may be set extraterrestrially?