EXPOSURE TO LONG SUMMER DAYS AFFECTS THE HUMAN MELATONIN AND CORTISOLRHYTHMS

Exposure of 8 human subjects in summer to a natural 16 h bright light photoperiod phase advanced the morning salivary melatonin decline and cortisol rise and shortened the nocturnal melatonin signal by 2 h rela tive to the winter patterns of the same subjects followed under a comb ined artificial and natural light 16 h photoperiod. The data suggest t hat summer days experienced from sunrise till sunset and not winter da ys with a combined artificial and natural light long photoperiod evoke a true long day response of the human circadian system.