Effects of light and sleep stages on heart rate variability in humans.

Effects of light intensity and sleep stages on heart rate variability (HRV) were investigated in young healthy subjects. The low-frequency (LF)/high-frequency (HF) ratio was significantly increased by exposing either to bright lights of 10 000 lx or to extreme darkness (< 0.01 lx), while HF and LF components of HRV were not changed, when compared with those under dim light (100 lx). However, LF was significantly increased at REM sleep, when compared with that at the pre-sleep wake. In contrast, HF was increased at all stages of sleep, and the LF/HF ratio was decreased at slow wave sleep during the baseline night.