Dichotomy in human population based on variability in peak spread of rest – activity rhythm in respect of internal phase reference point

Abstract

The present study was aimed at ascertaining the best reference point for the peak in rest – activity rhythm in a group of apparently healthy randomly chosen young university students consisting of nine males and seven females. The rest – activity was monitored by using an actiwatch (AW64, Mini Mitter Co. Inc., USA) at one-minute epoch length. Data were analyzed with the help of Actiware sleep software, Cosinor rhythmometry, SPSS software and Excel Analysis Toolpak. A statistically significant circadian rhythm in rest – activity was documented in all subjects, irrespective of gender. Further, the subjects were divided into two groups depending upon the variability in their computed peaks determined with reference to the conventional mid-night, start-sleep, mid-sleep and end-sleep timings. The peak spread in each case and in each subject was computed. Results indicate that mid-sleep and end-sleep timings produce peak spreads that are considerably shorter than the conventional phase reference timing, i.e., local mid-night. Thereafter, two distinct groups, namely “mid-sleep group” (MSG) with the least peak spread when the mid-sleep timing was taken as phase reference and “non-mid-sleep group” (NMSG) with the least peak spread when either start-sleep or end-sleep timing was taken as phase reference, were identified. Further a statistically highly significant difference between amplitudes in MSG and NMSG (p < 0.01) was obtained. A negative relationship was obtained between fragmentation index and amplitude in NMSG (r = −0.756). We have a hunch that there could be a phenotypical variation among the human population with respect to the mechanism of phase resetting. Furthermore, the amplitude could be a significant predictor of fragmentation index in the model that we fitted to elucidate an inverse relationship between them.

Keywords:

• Rest – activity
• peak
• phase reference point
• phenotypical variations

Acknowledgements

We are thankful to the Head, School of Life Sciences, Pt. Ravishankar Shukla University, Raipur. This study was supported by the University Grants Commission, New Delhi, and the Department of Science & Technology, New Delhi, through their DRS-SAP and FIST program, respectively. We are thankful to the subjects who participated in this study.