  Transitions in the dynamics of complex systems can be characterized by
changes in the synchronization behavior of their components. Taking the human
cardio-respiratory system as an example and using an automated procedure for
screening the synchrograms of 112 healthy subjects we study the frequency and
the distribution of synchronization episodes under different physiological
conditions that occur during sleep. We find that phase synchronization between
heartbeat and breathing is significantly enhanced during non-rapid-eye-movement
(non-REM) sleep (deep sleep and light sleep) and reduced during REM sleep. Our
results suggest that the synchronization is mainly due to a weak influence of
the breathing oscillator upon the heartbeat oscillator, which is disturbed in
the presence of long-term correlated noise, superimposed by the activity of
higher brain regions during REM sleep.
