Autonomic nervous system function is known to change from waking to sleep or during different sleep stages. Generally, parasympathetic tone is predominant during sleep, but sympathovagal balance varies according to the depths of sleep stages and types. Therefore, quantifying those measures related to autonomic functions is useful in assessing sleep.
Recently, a new method of quantifying sleep, termed cardiopulmonary coupling (CPC) analysis, was developed based on estimating the coupling of cardiac autonomic function and respiratory drives, using heart rate and respiratory modulation of QRS amplitude, respectively. Both informations can be read from a single channel of electrocardiogram. Physiologically stable sleep is associated with high-frequency coupling between heart rate and respiration at frequencies of 0.1 to 0.4 Hz, whereas physiologically unstable sleep is associated with low-frequency coupling between heart rate and respiration over a range of 0.01 to 0.1 Hz. The presence of very-low-frequency coupling between heart rate and respiration below 0.01 Hz is correlated with waking or REM sleep.
The CPC analysis has been validated to detect and phenotype sleep apnea based solely on the electrocardiogram signal. We have shown the use of CPC method in evaluating sleep quality in patients with major depression  and normal subjects . The results showed that depressed patients had significantly increased unstable sleep compared to healthy controls, and that this increase in unstable sleep can be partially normalized using hypnotic drugs.
Of interest, the spectral measures used in CPC analysis (i.e., low- or high-frequency coupling) are fundamentally different from conventional spectral heart rate variability analysis. In CPC analysis, we incorporate both respiration and heart rate signals to measure the degree of coupling between them.
1. Yang AC, Tsai SJ, Yang CH, Kuo CH, Chen TJ, Hong CJ.* Reduced physiologic complexity is associated with poor sleep in patients with major depression and primary insomnia. Journal of Affective Disorders 131:179-185 (2011).
2. Yang AC, Hong CJ, Kuo CH, Chen TJ, Yang CH, Li C, Tsai SJ. Profiling objective sleep quality in a healthy Taiwanese sample using a novel electrocardiogram-based cardiopulmonary coupling analysis. Taiwanese Journal of Psychiatry, 24:201-09 (2010).
Your comments and suggestions are welcome. Please send your comments by email to Dr. Albert C. Yang (Laboratory of Precision Psychiatry).