Sliding averages of MAP, SI and HR over a 6-hour period (courtesy of David Shannahoff-Khalsa, The Research Group for Mind-Body Dynamics, Institute for Nonlinear Science, University of California San Diego, California). The raw data were obtained in a beat-by-beat fashion during a sleep study and then subjected to the 200-beat sliding average calculation and plotting. These recordings document that in addition to the dynamic, beat-by-beat adjustments of HR to beat-by-beat variation of SI (as shown in Fig.2), the absolute value of HR continually adjusts to the absolute value of SI as well, in order to maintain CI at a level providing an adequate perfusion blood flow.