Measurement notes
Compute IS as the ratio of the variance of the average 24 h profile to the total variance of the record. Higher values mean the animal repeats the same daily pattern. Several days of data are needed for a stable estimate.
Endpoint methods library
Circadian & activity rhythm endpoint
Interdaily stability
How reproducible the 24-hour activity pattern is from one day to the next, quantified as interdaily stability (IS).
Unit
IS index 0 to 1
Readout
Interdaily stability (IS): ratio of the 24 h-profile variance to the overall variance
Assays
Home-cage video, beam-break, actigraphy
Decision summary
Use interdaily stability to ask how strongly behavior is coupled to the 24 h day and how repeatable the daily pattern is. IS runs from 0 (no day-to-day structure) to 1 (identical days) and drops with weak entrainment, aging, and circadian disruption. It pairs with intradaily variability: IS is between-day consistency, IV is within-day fragmentation.
| Primary value | Interdaily stability (IS): ratio of the 24 h-profile variance to the overall variance |
|---|---|
| Common units | Dimensionless index 0–1 |
| Compatible assays | Home-cage video, beam-break, actigraphy |
| Required boundary | Bin size and number of days (more days = more stable estimate) |
| Do not infer alone | Period, phase, or within-day fragmentation |
Interpretation limit
A high IS confirms a reproducible, well-entrained pattern but does not measure how strong the rhythm is within a day or whether the period is normal. Low IS can come from weak entrainment, environmental instability, or genuine clock dysfunction.
Data capture
Store animal ID, bin size, days analyzed, IS value, lighting condition, and activity measure.
Confound checks
- Too few days, which destabilizes the estimate.
- Day-to-day husbandry or lighting changes.
- Bin size choice.
- Masking that imposes apparent stability under LD.
Reporting checklist
- Number of days analyzed and bin size.
- IS definition and software.
- Lighting condition and activity measure.
- Paired IV and amplitude where reported.
References
Evidence notes
Endpoint pages should cite the method literature behind the scored value and keep high-specificity protocol claims qualified unless the source supports them.
- Witting W et al. Alterations in the circadian rest-activity rhythm in aging and Alzheimer’s disease. Biol Psychiatry. 1990. doi:10.1016/0006-3223(90)90523-5.
- Gonçalves BSB et al. Nonparametric methods in actigraphy: An update. Sleep Sci. 2014. doi:10.1016/j.slsci.2014.09.013.
- Van Someren EJW et al. Bright light therapy: improved sensitivity to its effects on rest-activity rhythms in Alzheimer patients by application of nonparametric methods. Chronobiol Int. 1999. doi:10.3109/07420529908998724.
Related workflow paths
Endpoint articles link to adjacent products, software workflows, and sibling endpoints where the connection is useful and already routable.
