Aging remodels the suprachiasmatic nucleus and weakens ultradian rhythms and activity–temperature coupling

Aging alters daily physiology, but how these changes relate to remodeling of the suprachiasmatic nucleus (SCN) remains incompletely understood. Here, we examined time-resolved SCN transcriptomic profiles alongside longitudinal recordings of core body temperature and locomotor activity, using separate cohorts for the molecular and physiological analyses. Aging was associated with extensive SCN transcriptomic remodeling, including enrichment of innate immune, interferon-related, complement-associated, and glial programs, while selected canonical clock genes retained detectable rhythmic expression. The composition and temporal profiles of broader rhythmic transcriptional programs also differed between young and aged SCN samples. Aged mice retained detectable circadian rhythms but showed reduced 2–6 h ultradian wavelet power in locomotor activity and body temperature, lower activity–temperature correlations, and altered optimal integration windows and time-of-day-dependent relationships. Histological analyses revealed glial remodeling and reduced neuronal marker-positive cell density in the aged SCN. These findings identify parallel age-associated changes in SCN molecular and cellular organization and in physiological rhythmic power and activity–temperature associations.