Length of one endogenous circadian cycle measured in constant darkness, the core readout of intrinsic clock speed.
Use free-running period when the question is whether a genotype, drug, or lesion changes the intrinsic speed of the circadian clock. Tau is defined only in the absence of entraining cues, usually constant darkness, and is estimated from the slope of activity-onset drift or from a periodogram. A normal mouse runs near 23.5–24 h; clock mutants commonly fall outside that range.
| Primary value | Period of the endogenous rhythm in constant conditions |
|---|---|
| Common units | Hours (e.g. 23.7 h) |
| Compatible assays | Running wheel, home-cage video, beam-break, telemetry under DD |
| Required boundary | Constant darkness window and estimation method (onset regression or periodogram) |
| Do not infer alone | Phase, amplitude, or entrainment ability |
Record continuously for one to three weeks in constant darkness, then estimate tau either by regressing daily activity onsets against day number or by taking the dominant peak of a chi-square or Lomb-Scargle periodogram. The two methods should agree within a few minutes for a robust rhythm.
Tau is only the free-running period when entraining cues are truly absent; residual light leakage, scheduled noise, or feeding can mask or entrain the rhythm and bias the estimate. A period near 24 h under a light-dark cycle reflects entrainment, not the endogenous period.
Store animal ID, DD start and duration, estimation method, periodogram parameters, tau with confidence interval, and excluded segments.
Endpoint pages should cite the method literature behind the scored value and keep high-specificity protocol claims qualified unless the source supports them.
Endpoint articles link to adjacent products, software workflows, and sibling endpoints where the connection is useful and already routable.
