Light and diurnal cycle affect human heart rate: Possible role for the circadian pacemaker

Humans and animals demonstrate diurnal rhythms in physiology and behavior, which are generated by the circadian pacemaker, located in the suprachiasma tic nucleus (SCN). The endogenous diurnal rhythm of the SCN is synchronized to the diurnal cycle most effectively by light. However, light also influe nces the SCN and its output instantaneously, as is demonstrated for the imm ediate effects of Light on SCN neuronal firing frequency and on the output of the SCN to the pineal, inhibiting melatonin secretion. In addition to th is, the circadian pacemaker modulates neuronally also other organs such as the adrenal. Therefore, the authors investigated the effect of this light i nput to the SCN on human heart rate, using Light at different phases of the day-night cycle and light of different intensities. Resting heart rate (HR ) was measured in volunteers between 20 and 40 years of age during supine, awake, resting conditions, and after 2 hours of fasting. In Experiment 1, H R was measured at different times over the day-night cycle at 0 lux and at indoor light. In Experiment 2, HR was measured at different times over the day-night cycle at controlled light intensities of 0 lux, 100 lux, and 800 lux. The authors demonstrate a clear diurnal rhythm in resting HR in comple te darkness, similar to that measured under constant routine conditions. Se cond, it is demonstrated that light increases resting HR depending on the p hase of the day-night cycle and on the intensity of light. These data stron gly suggest that the circadian pacemaker modulates human HR.