Older adults have an impaired blood pressure response to active standing, especially in the middle of the night; blame the circadian system

Introduction: Approximately 15 million older US adults report falling each year. Falls, especially those during the night, are a major cause of morbidity and mortality in this group. Such falls could be the consequence of failure of blood pressure (BP) recovery after standing up, which is an independent predictor of mortality. In younger adults, the circadian system causes an impaired BP response to passive tilt during the night but the circadian influence on BP responses to active standing is unknown. Thus, we compared the BP response to standing between healthy midlife and older adults. We hypothesized that older adults will have an impaired BP response to active standing, with the biggest impairment during the biological night. Methods: 25 healthy adults (19 between ages 40-59 years, and 6 between ages 60-70 years) completed a 5 day circadian ‘forced desynchrony’ protocol in continuous dim light (<3 lux), throughout which all behaviors and measurements were evenly distributed across the 24-hour cycle. Participants underwent 10 recurring 5 h 20 min cycles of 2 h 40 min sleep opportunities and 2 h 40 min of standardized wake periods. After 24 minutes of supine rest at the beginning of each waking period, we performed a 5-minute active standing test and measured BP and heart rate (HR) at 1 and 3 minutes into standing. Circadian phases of measurements were determined relative to the dim light melatonin onset, calculated from sequential salivary samples. Circadian variations between groups in the BP and HR changes upon standing were tested using mixed model designs with circadian phase as a fixed factor and subject as a random factor with significance set as p<0.05. Results: Compared to midlife people, on average, older adults have a significantly impaired systolic BP response at 1 min (-12 mmHg, p=0.014) and 3 min (-11 mmHg, p=0.03) and an impaired diastolic BP response at 3 min (-6 mmHg, p=0.013), all without a significant difference between groups in HR responses throughout the 24 h circadian cycle. The biggest impairment in systolic BP was noted during circadian phases corresponding to ~10:30 to 6:30 AM (-12 to -19 mmHg difference compared to midlife adults). Conclusion: In healthy old adults, the circadian system contributes to a dramatically impaired BP response to active standing, mainly during the biological night. This impaired response could lead to an increased risk of falls during the night in this group. We speculate that since the changes in HR were not significant and diastolic BP response also declined at 3 minutes, the overall impairment is more likely to be driven by compromised peripheral vasoconstriction than central mechanisms. NIH R01HL125839, R01HL163232; R35HL155681, OCTRIUL1TR002369, and Oregon Institute of Occupational Health Sciences. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.