THE body鈥檚 natural clock influences much more than just our sleep-wake patterns and hormonal cycles. Physicists have found that circadian rhythms affect the variability of heartbeats and even the movement of limbs.
It has long been known that heart attacks and other heart-related failures occur most often in the hours around 10 am, presumably reflecting the stress of morning activity following a night of rest. But Eugene Stanley鈥檚 team from Boston University in Massachusetts discovered that circadian rhythms might also have a role to play. They studied the variability in the time between heartbeats in different people. Lower variability has been linked to heart disease. 鈥淚ndividuals with a regular heartbeat, statistically, are likely to have a less healthy heart,鈥� says Stanley.
The team found that circadian rhythms appear to influence this variability: the unhealthiest heartbeat patterns appear around 10 am, regardless of whether a person is awake or asleep, active or inactive. It is possible that the body鈥檚 clock pushes already weak hearts into the danger zone, says team member Plamen Ivanov.
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The researchers also recorded for the first time the influence of circadian rhythms on motor activity. They fitted volunteers with wristwatch-like sensors to measure forearm motion at different times of day and night. They found that arms accelerated most rapidly in the afternoon, regardless of what physical activity the volunteers were engaging in. 鈥淲e find that the activity level is not random, and this may provide some useful information for clinical use,鈥� says team member Kun Hu.
Ivanov says the circadian influences on heartbeat and arm motion appear to reflect deep links in the way the nervous system controls body functions.
