Light-dark cycles and behavioural activity influence daily regulation of cardiovascular function. To avoid overinterpretation, the present study specifically examined day-night variation under a 12:12 h light-dark schedule rather than endogenous circadian rhythmicity. The STE20/SPS1-related proline-alanine-rich kinase (SPAK) is a key regulator of ion transport and blood pressure, but its association with day-night cardiovascular variation remains unclear. Using radiotelemetry, we assessed the effects of light-dark phase, activity state, and recording cycle on cardiovascular parameters in SPAK knock-in (KI) and wild-type (WT) mice. Heart rate, systolic, diastolic, mean arterial, and pulse pressure were recorded across three consecutive day-night cycles. Repeated-measures ANOVA was performed using animal as the unit of analysis. In WT mice, phase effects were not statistically significant for most parameters, although some effect-size estimates were notable. In contrast, KI mice showed significant light-dark variation across cardiovascular parameters in both mean and area-under-the-curve analyses (all < 0.05). Activity state strongly influenced heart rate and blood pressure in both genotypes, whereas pulse pressure was less consistently affected. Cycle effects were generally not significant, suggesting no clear evidence of systematic drift across recording windows. However, interpretation is limited by the exploratory nature of the study and the small sample size ( = 3/group), which increases the risk of both Type I and Type II statistical errors. These findings suggest that the SPAK knock-in genotype is associated with enhanced day-night modulation of cardiovascular function under a 12:12 h light-dark protocol, while activity state remains a major determinant of cardiovascular variability. p n
