Heart rate variability -based recovery during sleep compared to movement analysis,subjective ratings and cortisol awakening responses

Tero Myllymaki. Heart Rate Variability –Based Recovery during Sleep Compared to Movement Analysis, Subjective Ratings and Cortisol Awakening Responses. 107 p. Master’s Thesis in Exercise Physiology. Department of Biology of Physical Activity, University of Jyvaskyla. Prolonged work stress of employees or heavy training without adequate recovery of athletes can alter the balance between the sympathetic and the parasympathetic branch of the autonomic nervous system (ANS) towards sympathetic dominance, which is a risk factor for cardiovascular health. Heart rate (HR) and heart rate variability (HRV) recordings have been used as indicators of the function of ANS. A new method has been developed to evaluate stress and recovery from ambulatory RR-interval recordings. Firstbeat PRO Heartbeat Analysis Software (FBT-PRO) combines information on HR and HRV indices to evaluate the state of the body, i.e. whether the body of a person is e.g. under stress reactions, recovering (in relaxation state), or doing physical exercise. The aims of this study were to assess stress and relaxation during awake and sleep, and to compare FBT-PRO indices during sleep to indices of movement based Actiwatch Sleep Analysis (ASA), to subjective ratings of the quality of sleep, and to cortisol awakening responses, which all have been used a lot in stress research. The subjects were 14 nurses from the Central Hospital of Central Finland (age: 44±10 years, height: 163±5 cm, and weight: 63±10 kg). RR-intervals and wrist movements were recorded for 24 hours during workday 1 (WD1), workday 2 (WD2) and day off (DO) and analyzed for stress and relaxation with FBT-PRO and for sleep quality with ASA, respectively. The subjects took three saliva samples (at awakening, 30 min and 60 min after) each morning for cortisol analyses (raw values and cortisol awakening responses during 60 minutes from awakening) and rated whether they had slept well, moderately or badly (1–3). The two workday sleeps were categorized into better or worse sleep based on subjective ratings, fragmentation of the sleep, actual sleep time (AST), cortisol at awakening, and cortisol awakening response. In addition, HRV variables were analyzed with time and frequency domain methods and calculated for each 5-min segment. Nonparametric Wilcoxon and Marginal Homogeneity tests were used in the statistical analysis. Significantly more stress during awake (65±20% vs. 25±19%, p=0.008) and relaxation during sleep (1±3% vs. 65±19%, p=0.002) was detected with FBT-PRO. During sleep, HR was lower (84±6 vs. 60±5 btm, p=0.002), and RMSSD and HF power greater (p=0.003 for both), whereas LF/HF ratio was lower (p=0.002) than during awake. AST was significantly longer (6:34±1:05 vs. 7:55±0:41 h:min, p=0.005), and there was more relaxation time detected (4:29±1:38 vs. 5:25±1:58, p=0.037) during DO than during WD2 sleep, which did not differ from WD1 sleep. The DO sleep was also rated to be subjectively better (p=0.034) than WD2 sleep. When better and worse workday sleeps were compared, there were no differences in stress or relaxation during subjectively different sleeps. Relaxation time was longer during less than more fragmented sleep (5:05±2:01 vs. 4:35±1:48 h:min, p=0.026). Relaxation time was longer (5:06±1:37 vs. 4:25±1:37 h:min, p=0.026) and there was greater RMSSD (p=0.050) during workday sleep with longer than with shorter AST. The increase and change in cortisol after awakening (measured with area under curve) were smaller (p=0.050 for both) during workday sleep with longer than with shorter AST. Lower awakening cortisol was accompanied by a greater increase and change in cortisol (p=0.050 and 0.041, respectively), and there tended to be less stress time (p=0.075) during sleep with lower awakening cortisol. There was longer AST (p=0.050) and greater sleep efficiency measured with ASA (p=0.029) but no differences in stress or relaxation variables during sleep with a smaller cortisol awakening response. The results show that stress and relaxation states can be detected from HR and HRV, i.e. there was more stress during awake and more relaxation during sleep. Parasympathetic modulation seems to be dominating sleep and sympathetic activity awake-time. During workdays with longer actual sleep time and during day off, more relaxation time during sleep was achieved through longer sleep. Restlessness during sleep was related to reduced relaxation time and to a shift in favour of stress in relaxation/stress balance. Sleep with lower awakening cortisol tended to include less stress time, but cortisol awakening response did not seem to differentiate stress or relaxation times during sleep. These results give further evidence that both sleep quantity and quality are important factors for adequate physiological recovery. In conclusion, the new HR and HRV –based method seems to allow the analysis of stress and recovery during sleep and awake in healthy working population. More studies are needed with greater number of subjects and with different subject groups (e.g. athletes) to find out more specifically the connections between this new method and the other methods used in measuring recovery.