Opposite Seasonal Variation in Nighttime and Morning Blood Pressure: A Longitudinal Study of Patients with Hypertension

Abstract Background Longitudinal data on seasonal variations in nighttime blood pressure (BP) obtained through home blood pressure monitoring (HBPM) are scarce. We examined whether nighttime BP is elevated in summer compared to winter in patients undergoing hypertension treatment. Methods Nighttime BP was measured for seven days using an HBPM device in 419 participants at baseline, and 136 of those underwent follow-up nighttime BP measurements at 6 months without any modification of antihypertensive therapy. Morning BP was also measured in some participants. Results The baseline cross-sectional analysis indicated that nighttime systolic BP was elevated in summer compared to winter (adjusted mean difference, 8.3 mmHg; 95% confidence interval [CI], 4.7-11.8). The longitudinal analyses revealed that nighttime systolic BP was higher in summer than in winter among winter-to-summer participants (mean difference, 5.1 mmHg; 95% CI, 3.1-7.0; n = 36) and among summer-to-winter participants (mean difference, 2.6 mmHg; 95% CI, 0.3-5.0; n = 23). Conversely, morning systolic BP was lower in summer than in winter among winter-to-summer participants (mean difference, 3.8 mmHg; 95% CI, 0.7-7.0) and among summer-to-winter participants (mean difference, 3.7 mmHg; 95% CI, 1.0-6.4). In sensitivity analyses, seasonal variations in room temperature were positively correlated with changes in nighttime systolic BP (r = 0.43; 95% CI, 0.21-0.60) and negatively correlated with changes in morning systolic BP (r=–0.46; 95% CI, –0.63 to –0.25). Conclusions Nighttime systolic BP increased from winter to summer in association with higher room temperature and exhibited an inverse seasonal pattern compared to morning BP.