Heart-rate variability in reversed slow-wave/REM sleep patterns during an Antarctic summer expedition

Introduction: Sleep-wake regulation, physical activity, circadian rhythms, mood and performance were investigated during a 4 month summer expedition in Antarctica: the BELARE (Belgian Antarctic Research Expedition) campaign 2008-2009. Results with regard to previously cited variables included a phase delay of melatonin secretion (peaking around 07.00 AM), as well as a delayed occurrence of slow wave sleep (SWS), and a severe decrease in SWS, paired with an increase in REM sleep. Here we present the investigation of nocturnal cardio-respiratory regulation and heart rate variability. Method(s): Data were collected every 2 weeks for each subject. These included 48 h actigraphy, one night polysomnography, morning and evening Profile of Mood States and Karolinska Sleepiness Scale, morning Psychomotor Vigilance Test. Morning and evening saliva samples were taken to determine melatonin levels. Circadian rhythms profiles were determined with a one-time 18 h profile (sampling every 2 h) of cortisol and melatonin. ECG and respiration (thoracic and abdominal) were continuously recorded with the LifeShirt system (Vionoetics, Inc) at 200 Hz, digitally upsampled at 1 kHz. Frequency-domain analysis of heart variability was performed parallel with standard sleep staging with Fast Fourier Transform. Result(s): Results show an average R-R interval (RRI) of 896 ms, and median RRI of 992 ms, with a reduced distribution (upper quartile range: 1046 ms; lower quartile range: 864 ms). Breathing frequency (Fresp) showed to be surprisingly high (mean Fresp: 18,3 min)1; median Fresp: 13,5), with a high occurrence of intermittent periodic breathing. Heart rate variability analysis and parallel sleep stage/ cardio-respiratory analysis is still ongoing to further investigate whether (i) the disturbed breathing pattern is related to the disturbed sleep pattern and (ii) heart rate variability shows the usual sympathetic activation during REM sleep and parasympathetic increase during SWS. Conclusion(s): Both sleep patterns and cardio-respiratory regulation are altered during sleeping period in the Antarctic summer. The delayed and decreased SWS could be subtended by the phase delay in melatonin secretion. Further analysis will show whether a modified autonomic regulation is related, or independent from these findings.