Effects of bright light and melatonin on sleep propensity, temperature, and cardiac activity at night

Melatonin increases sleepiness, decreases core temperature, and increases p eripheral temperature in humans. Melatonin may produce these effects by act ivating peripheral receptors or altering autonomic activity. The latter hyp othesis was investigated in 16 supine subjects. Three conditions were creat ed by using bright light and exogenous melatonin: normal endogenous, suppre ssed, and pharmacological melatonin levels. Data during wakefulness from 1. 5 h before to 2.5 h after each subject's estimated melatonin onset (wake ti me +/- 14 h) were analyzed. Respiratory sinus arrhythmia (cardiac parasympa thetic activity) and preejection period (cardiac sympathetic activity) did not vary among conditions. Pharmacological melatonin levels significantly d ecreased systolic blood pressure [5.75 +/- 1.65 (SE) mmHg] but did not sign ificantly change heart rate. Suppressed melatonin significantly increased r ectal temperature (0.27 +/- 0.06 degreesC), decreased foot temperature (1.9 8 +/- 0.70 degreesC), and increased sleep onset latency (5.53 +/- 1.87 min) . Thus melatonin does not significantly alter cardiac autonomic activity an d instead may bind to peripheral receptors in the vasculature and heart. Fu rthermore, increases in cardiac parasympathetic activity before normal nigh ttime sleep cannot be attributed to the concomitant increase in endogenous melatonin.