Analysis of heart rate variability. Several factors produce inconsistent heart beat cycles. Under resting conditions, the cardiac interbeat signal fluctuates probably because of thermal and blood pressure control systems.
Respiration also causes cycle fluctuations.
In mental work, it is the change in respiratory pattern and not rate, that causes the HR characteristics. HR is not a valuable parameter for exploring the workload response in a normal individual. A respiratory pattern is the duration, depth, and detailed shape of the intrapleural pressure wave form. Such a spectrum is revealed over a 60-120 sec period. HR may be more stable at higher rates.