Effects of infrared radiation on the diurnal rhythms of melatonin,rectal temperature and heart rate

Abstract

Infrared radiation presented in the early night was expected to delay the circadian rhythm via suppression of melatonin synthesis. Seven healthy men (16–22 yrs) had completed a constant routine (24-h bedrest, <30 lux, 18°C) and 2 experimental sessions where bright light (BL, 1 500 lux) or infrared radiation (IR, 65°C) was applied from 1700 to 0100 h. Salivary melatonin levels were determined hourly, rectal temperature and heart rate continuously. Melatonin synthesis was suppressed by BL; the onset and the maximum of the melatonin profile and the minima of rectal temperature and heart rate were then delayed thus indicating a circadian effect. IR did not affect melatonin concentration whereas the minima of rectal temperature and heart rate were delayed indicating a dissociation due to the heat-induced elevation of both the latter variables rather than a chronobiologic effect. This study does not indicate that infrared radiation affects the circadian rhythm in night shift workers.