Infradián

Infradián is a term encountered in some chronobiology discussions to denote rhythmic processes with periods shorter than 24 hours. The label is not standardized, and its usage varies across languages and subfields. In many contexts, ultradian rhythms are the more widely accepted term for subdaily cycles, while infradián is sometimes treated as a historical or informal synonym. Because of its inconsistent application, infradián can be confused with infradian rhythms, which refer to cycles longer than a day.

Most rhythms labeled as infradián-like have periods ranging from minutes to several hours. Examples often cited in the literature include hormonal pulsatility (such as bursts of certain hormones), neural oscillations, cardiovascular and respiratory variability, and elements of sleep architecture that recur within a night. These subdaily cycles can be endogenously generated by central and peripheral clock mechanisms or be influenced by external cues such as feeding, activity, and light.

Measurement and interpretation of infradián phenomena rely on time-series analysis and chronobiological methods, including actigraphy, polysomnography, hormone profiling, and spectral or cosinor analyses. Because the term lacks universal acceptance, researchers typically prefer ultradian or related descriptors, depending on the exact period and physiological context. As terminology evolves, infradián remains a label used primarily in certain discussions and texts, serving as a reminder of the broader spectrum of biological rhythms beyond circadian cycles.