The free-running circadian period, τ (tau), is the intrinsic period of an organism's biological clock measured in the absence of external time cues such as light-dark cycles. Under constant conditions — typically constant darkness (DD) or constant light (LL) — circadian rhythms persist with a period that reflects the endogenous pacemaker. In most organisms τ is close to, but not exactly, 24 hours, and it varies across taxa, life history stages, and genetic backgrounds.

This database compiles free-running period measurements from the primary literature spanning bacteria, fungi, plants, protists, and animals. Records include the mean τ, experimental light condition, species identity, genotype, and source citation. Use the Plots tab to explore distributions across taxa and conditions, or the Data tab to search and filter individual records.