interdaily

Interdaily is an adjective meaning occurring between days or from one day to the next. In chronobiology and sleep research, interdaily often appears as part of measures that assess how regular or stable a person’s daily rhythms are across multiple days. The most common related term is interdaily stability (IS), which quantifies the degree to which an individual’s 24-hour activity-rest pattern is reproducible from day to day. IS is typically derived from continuous monitoring data such as actigraphy or other wearable sensors that record activity, light exposure, or related cues over several days.

Interdaily measures are used to characterize circadian rhythm organization in various populations, including healthy adults, older adults, shift workers, and patients with sleep or mood disorders. A high interdaily stability value indicates a consistent, predictable pattern across days, suggesting a well-entrained or stable circadian system. Lower interdaily stability suggests greater day-to-day variability in the timing or shape of daily activity and rest, which can reflect irregular schedules, disrupted routines, or underlying health issues.

Data requirements for assessing interdaily stability include multi-day, high-quality time-series recordings with sufficient sampling resolution. Researchers may combine objective measures (actigraphy) with subjective sleep diaries to improve accuracy. Interdaily analysis is often complemented by other rhythm metrics, such as intradaily variability, which reflects fragmentation within days, to provide a fuller picture of circadian patterning.

See also circadian rhythm, sleep-wake cycle, actigraphy, and chronobiology.