Studying five healthy volunteers for 10-day periods in pioneering efforts
to ultimately answer this question, a collaboration of Boston University
physicists and Harvard physiologists has found evidence that the body's
circadian clock (a part of the brain that regulates daily biological
activities) influences patterns in the heart's "interbeat intervals,"
the lengths of time between successive heartbeats. At around 10AM for
all the healthy individuals, the values of successive interbeat intervals
displayed increased signs of randomness, statistically resembling that
seen in previous studies of individuals with heart disease.
In their studies, the researchers took special care to isolate the effects
of a person's internal circadian clock (which has a 24.2-hour rhythm,
marked by a regular rise and fall of body temperature) from the effects
of behavior (such as physical activity and a person's wake/sleep time)
or external stimuli (such as the rising or setting of the sun). Towards
these ends, the researchers made sure to "desynchronize" the individuals'
internal body clocks from these other factors by keeping the volunteers
in a dimly lit room and by varying their sleep and wake times from day
to day while keeping activity levels relatively constant.
The researchers next plan to explore how an individual's behavior may
interact with the circadian clock to influence the correlations in interbeat
intervals. The researchers have not yet studied patients with heart
disease and are far from being able to make clinical recommendations.
However, their further research may obtain insights into the underlying
causes of increased cardiac risk and could lead to improved therapy,
such as more appropriately timed medication to coincide with phases
of the body clock. (Hu et al., Proceedings
of the National Academy of Sciences, December 28, 2004; contact
Plamen Ch. Ivanov, Boston University, 617-353-3891, firstname.lastname@example.org;
Steven Shea, Harvard Medical School, 617-732-5013, email@example.com)