ness with eight hours of scheduled sleep) for two to three weeks to stabi‐ lize circadian rhythms. They then underwent two baseline days and nights in the laboratory with the same sleep‐wake schedule as at home, fol‐ lowed by a prolonged laboratory pro‐ tocol designed to separate internal cir‐ cadian effects from behavioral and environmental effects on blood pres‐ sure and other physiological variables. These controlled behavioral and envi‐ ronmental conditions included activity, posture, meals, sleep, room tempera‐ ture, and light. The researchers mea‐ sured core body temperature to track circadian time. The three protocols, carried out in dim light to avoid reset‐ ting any of the body’s circadian rhythms, included: · 38‐hour “constant routine,” with continuous wakefulness and con‐ stant body posture.
· 196‐hour “forced desynchrony” (forcing a sleep/wake cycle to con‐
flict with participants’ normal pat‐ tern) consisting of recurring 28‐ hour sleep/wake cycles with 18 hours, 40 minutes awake and 9 hours, 20 minutes asleep.
· 240‐hour “forced desynchrony” with twelve recurring 20‐hour sleep/wake cycles.
All three protocols revealed almost identical systolic and diastolic circadian rhythms, which were unrelated to other internal rhythms that are known to influence cardiovascular events, such as increases in sympathetic ner‐ vous system activity.
The reason for the peak in heart attacks and strokes in morning hours remains unexplained by these findings, but could certainly be related to the activities that normally occur on a regu‐ lar daily basis. “For example, other recent work by our group indicates that the body clock interacts with behaviors, such as exercise, to cause an exaggerated increase in cardiovascular
risk markers during the biological morning,” adds Frank A. Scheer, Ph.D., co‐author of the study and assistant professor of medicine at Harvard Medical School. It is possible that the evening peak in the internal circadian blood pressure cycle may help to explain a second, albeit less prominent increase in heart attacks that occurs in the evening, the researchers said. “We now need to study people with different vulnerabilities and risk factors for cardiovascular disease,” Shea said. “So far, we have studied very healthy people who don’t have hypertension or the build‐up of arterial plaques that could bring them closer to the theoret‐ ical threshold for precipitating an adverse cardiac event.”EMS
Other co-authors are Michael F. Hilton, Ph.D.; Kun Hu, Ph.D.; and Frank A.J.L. Scheer, Ph.D. Author disclosures and funding are on the manuscript.
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