  We test whether the complexity of cardiac interbeat interval time series is
simply a consequence of the wide range of scales characterizing human behavior,
especially physical activity, by analyzing data taken from healthy adult
subjects under three conditions with controls: (i) a ``constant routine''
protocol where physical activity and postural changes are kept to a minimum,
(ii) sympathetic blockade, and (iii) parasympathetic blockade. We find that
when fluctuations in physical activity and other behavioral modifiers are
minimized, a remarkable level of complexity of heartbeat dynamics remains,
while for neuroautonomic blockade the multifractal complexity decreases.
