Circadian Rhythms In Blood Glucose And Blood Pressure

Abstract

Circadian rhythms are physiologic fluctuations over a 24-hour period. Two variables thought to demonstrate a circadian rhythm include brachial blood pressure and blood glucose. However, there is great variation among the protocols in the current literature, and a lack of congruity on time-points chosen for measurement. PURPOSE: To examine potential rhythms of systolic (SBP), diastolic (DBP) blood pressure and blood glucose without interrupting an individual’s natural sleep/wake cycle. A second aim was to determine if these potential rhythms are reproducible. METHODS: After establishing a normal waking time, 7 strength trained participants scheduled their first “Circadian testing” day. The first measurement of SBP, DBP and blood glucose were taken approximately 2 hours after their normal wake time. Measurements were repeated every 2 hours for 12 hours, while participants rested. To examine the repeatability of the potential rhythm, participants returned and completed the same procedures as before within 14 days of their first “Circadian testing” visit. Food and drink intake was recorded during the first visit and participants consumed the same diet on the second day of “Circadian Testing”. A 2x7 (Day x Time) repeated measures ANOVA was used to determine if differences existed for SBP, DBP, and blood glucose. Significance was set at p ≤ .05. Data are presented as mean ± SD. Cosinor analysis was used to determine rhythm patterns for each variable each day RESULTS: There was no interaction (p=0.751) or time main effect (p=0.236, range: 115-120 mmHg) for SBP. There was a main effect of day, with SBP being lower on Day 2 than Day 1 [116 (7) vs. 119 (5) mmHg, p=0.018]. No differences were observed with DBP. For blood glucose, there was no interaction (p=0.844), time main effect (p=0.613, range: 95.1-103.7 mmol/L), or main effect of day [Day 1: 98.7 (7) vs. Day 2: 97.5 (4) mmol/L, p=0.501]. Cosinor analysis revealed that there was no rhythm for blood pressure or blood glucose across time for either day (p>0.05) CONCLUSIONS: When accounting for an individual’s natural sleep/wake cycle, no measurable rhythms for SBP, DBP, or blood glucose were detected. Given this finding was observed twice, suggests that these hypothesized intrinsic rhythms may be masked by different zeitgebers or, perhaps, less pronounced when not disturbing sleep.