PHYSICAL STRESS IN THE MIDDLE OF THE DARK PHASE DOES NOT AFFECT LIGHT-DEPRESSED PLASMA MELATONIN LEVELS IN HUMANS

The human pineal gland has been shown to be unresponsive to stress-ind uced sympathetic activation during the day. However, the effects of st ress on human melatonin production have received little investigation at night, when the pinealocytes should be physiologically responsive t o noradrenergic stimulation. For this purpose, plasma melatonin and co rtisol levels were measured in 7 healthy men (aged 25-34 years), both in resting condition and before and after a physical exercise performe d between 23.40 and 24.00 h, 30 min after exposure to bright light (2, 500 lx). The exercise consisted in bicycling on a bicycle ergometer at 50% of the personal maximum work capacity (MWC) for 10 min, followed by another 10 min of bicycling at 80% of the MWC. The results clearly showed that physical exercise does not affect light-depressed plasma m elatonin levels, whereas it clearly increased plasma cortisol concentr ations (p < 0.002, two-way ANOVA with repeated measures), systolic blo od pressure, pulse pressure and heart rate. These findings suggest tha t the human pineal gland is not responsive to systemic sympathetic act ivation induced by physical stress even in the middle of the dark phas e.