Falls affect approximately 30% of older adults and are a major cause of morbidity and mortality, especially when they occur at night. Impaired blood pressure (BP) recovery upon standing is a known predictor of falls and mortality. Whereas younger adults show circadian-system-driven reductions in BP response to passive postural tilt at night, circadian influences on BP responses to active standing in both young and older adults are unknown. Twenty-five healthy adults (19 midlife, 50 ± SD 5 years; 6 older, 64 ± 2 years) completed a 5-day laboratory protocol in dim light where all behaviours and all physiological measurements (e.g., cardiovascular response to active standing) were evenly distributed across the 24-h period. Participants underwent 10 recurring 5-h 20-min behavioural cycles of 2-h 40-min sleep opportunities and 2-h 40-min standardized waking episodes. Active standing challenges occurred during each wake period, and BP and heart rate were measured at rest and at 1 and 3 min of standing. Older adults had significantly impaired systolic BP responses at 1 min (P = 0.014) and 3 min (P = 0.03) into standing, particularly during the biological night (10:30 PM to 6:30 AM). Diastolic BP was also significantly reduced at 3 min (P = 0.013) in older adults. Heart rate responses were elevated on standing but did not differ significantly between groups or across circadian phases. Results were independent of body mass index, resting BP, and sex. The endogenous circadian system impairs BP response to standing in healthy older adults at night, potentially increasing fall risk. Chronotherapy strategies may include adjusted medication timing, compression garments, or safer nighttime mobility options. KEY POINTS: About 30% of older adults experience falls, which are a leading cause of injury and death. Blood pressure (BP) maintenance after standing is key to maintaining brain blood flow and is a predictive factor of future falls and death. We discovered that in older adults, the internal body clock-related mechanisms impair BP responses when they stand, particularly during the biological night. Compared to midlife adults, in older adults, BP decreased significantly during the night during standing, and did not recover for 3 min after active standing, without a compensatory increase in heart rate. These changes increase the risk of fainting and falls in older adults. Our data support the use of better-timed BP medications to maintain nighttime BP in older adults.
