Strength of the day-night difference in activity, quantified as relative amplitude (RA) between the most and least active hours.
Use circadian amplitude to ask how strong or robust the rhythm is, separate from its timing or period. The nonparametric relative amplitude (RA) compares the most active hours with the least active hours and runs from 0 (no day-night difference) to 1 (maximal separation). It falls in aging, neurodegeneration, and circadian disruption even when period is unchanged.
| Primary value | Relative amplitude: (M10 − L5) / (M10 + L5) from the most-active 10 h and least-active 5 h |
|---|---|
| Common units | Dimensionless index 0–1, or raw activity difference |
| Compatible assays | Home-cage video, beam-break, wheel, actigraphy |
| Required boundary | Averaging window (days) and bin size |
| Do not infer alone | Period, phase, or absolute activity level |
Compute RA from an averaged 24 h activity profile: take the mean of the most active 10 hours (M10) and the least active 5 hours (L5), then RA = (M10 − L5)/(M10 + L5). Spectral amplitude from a cosinor or FFT fit is an alternative parametric measure.
A low amplitude means a weak or disrupted rhythm but does not by itself say whether the clock is slow, mistimed, or fragmented. Amplitude is sensitive to overall activity level and to masking, so pair it with period and a fragmentation measure.
Store animal ID, lighting condition, bin size, days averaged, M10, L5, RA, and the activity measure used.
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.
