Chronobiologic optimization of exercise physiology and practice guided by heart rate variability

For developing a marker rhythm-based protocol to determine whether the benefits from exercise training can be optimized by timing, a clinically healthy 24-year-old woman monitored her heart rate at 1-minute intervals for about 24 hours, using a relatively inexpensive instrument. A circadian rhythm peaking in the afternoon is demonstrated for heart rate and for heart rate variability (P < 0.001), gauged by the standard deviation computed over 5 minutes. Least squares spectra of heart rate reveal the predominance of low over high frequency components. Since heart rate variability is a putative index of autonomic cardiovascular function, and since exercise training can improve heart rate variability in patients at increased vascular disease risk, the time structure (chronome) of heart rate and heart rate variability could cost-effectively guide the assessment and optimization of exercise training.