IF YOU thought sleep is when you really relax, think again. Scientists have discovered that the heart comes under especially close control during sleep. Their findings suggest that sleep may play a role that until now has remained hidden.
Scientists now know that the heartbeat is far more complex and irregular than once supposed (New Scientist, 3 January 1998, p 20). The signals that control a healthy heartbeat contain many different frequency components, and this leads to a highly complex pattern of intervals between beats.
The gaps between heartbeats are "anti-correlated": if the heart speeds up over a short sequence of beats, it will slow down again over the next few beats. More importantly, the pattern of these speed-up/slow-down events can be seen on both short and long timescales: it looks the same over a few beats as it does for a thousand beats. In other words, the heartbeat has a fractal pattern.
An international team led by Plamen Ivanov of the physics department at Boston University decided to compare how the fluctuations in the interval between beats varied between day and night. The results surprised them: during the supposedly relaxed state of sleep, the intervals between heartbeats fluctuated much more strongly, but were brought under control more quickly, showing that the heart is under much closer control during sleep. "It was totally counterintuitive," says Ivanov.
The team initially thought the difference might come about because the body is resting during sleep. Physical activity tends to cause changes in the heartbeat, which would lead to more fluctuations and give the appearance that the heart had less control. But a marked difference remained even when these external influences were taken into account.
The researchers then compared their results with heart data from the cos-monauts on Mir. "It was interesting to compare them with the cosmonauts who were doing something different under different conditions," says Luis Amaral of the Boston team. "But we still got the same results, so this is definitely picking up something intrinsic."
In people with heart disease, the heart is under weaker control than in healthy people. But the researchers found that even heart patients showed an increased level of control when they were asleep.
The results suggest that there might be different mechanisms controlling the heart during waking and sleep, hinting at a hitherto unsuspected role for sleep. "I can't tell you what sleep means," says Harvard cardiologist Aryl Goldberger, "but maybe a function of sleep is to readjust the heart by exercising the system."
Author:Jens Thomas New Scientist magazine issue 4th Dec. 99 Source: Europhysics Journal (vol 48, p 594)
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