Physiological reaction to light exposure
arrhythmia, ’
ARRHYTHMIA
Measures of heart rate variability (HRV) reflect the activity of the sympathetic and parasympathetic branches of the autonomic nervous systemTo study the light-induced cardiovascular responses, an electrocardiogram was continuously recorded on each study day using the portable device ‘Mega Faros 180’ (Bittium Corporation, Finland). The ‘Mega Faros 180’ is waterproof (IP 67), lightweight (13 g), tiny (48 × 29 × 12 mm), and provides a high-resolution (up to 1024 Hz) single-channel electrocardiogram (ECG). In the present study, the electrocardiogram was stored at a sampling frequency of 1000 Hz. During preparations at the beginning of this study, we noted an inaccuracy of the real-time clock of the ECG device of several seconds when data were recorded over a period of six or more hours. As countermeasures, we (1) synchronized the ECG device clock with the computer clock by initializing it at the beginning of each study day and (2) instructed study participants to press a marker button on the device at the beginning of each cognitive test to quantify the discrepancy in the clock times of the ECG and attentional test data streams. Raw data were imported into the software ‘Cardioscope Analytics’ (Smart Medical, Gloucestershire, United Kingdom) and analyzed using a 5-min moving-window technique, with heart rate data analyzed in 5-min windows that followed each other in 30-s epochs. For each epoch, a quality index was generated by the software (incorporating beat and arrhythmia abnormalities). In the present study, a quality index of at least 95% was set as the lower threshold. To determine HRV, the R-waves of the QRS signal in the electrocardiogram were first detected by the software, and then the time between two consecutive R-waves (RR interval; in milliseconds) was determined. Based on the RR intervals, the root mean squared of successive difference in RR intervals (RMSSD) was determined as a robust indicator of HRV.Based on the gliding 5-min window technique, RMSSD data for continuous 30-s epochs were available and had to be prepared for data analysis. To examine the temporal dynamics of physiological responses during the performance of aPVT, HRV parameters were first averaged for each subject over the three separate 5-min periods of aPVT (termed ‘baseline’, ‘intervention’, and ‘different light’) and these data were subjected to statistical analyses. For the 10-min GNT, the individual HRV parameters derived from the central 5-min test periods (minute 2.5 to minute 7.5) were averaged for each subject and then subjected to statistical analyses. A similar procedure was applied to the HRV parameters derived from the five 25-min resting periods. Here, the individual RMSSD data from the central 20-min period (minute 2.5 to minute 22.5) were averaged and then subjected to statistical analyses.