  We compare scaling properties of the cardiac dynamics during sleep and wake
periods for healthy individuals, cosmonauts during orbital flight, and subjects
with severe heart disease. For all three groups, we find a greater degree of
anticorrelation in the heartbeat fluctuations during sleep compared to wake
periods. The sleep-wake difference in the scaling exponents for the three
groups is comparable to the difference between healthy and diseased
individuals. The observed scaling differences are not accounted for simply by
different levels of activity, but appear related to intrinsic changes in the
neuroautonomic control of the heartbeat.
