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 relative to the winter patterns of the same subjects followed under a combined artificial and natural light 16 h photoperiod. The data suggest that summer days experienced from sunrise till sunset and not winter days with a combined artificial and natural light long photoperiod evoke a true long day response of the human circadian system.

Institute of Physiology Academy of Sciences of the Czech Republic Prague

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From the Pineal Gland to the Central Clock in the Brain: Beginning of Studies of the Mammalian Biological Rhythms in the Institute of Physiology of the Czech Academy of Sciences