ABSTRACT
Melatonin onset was defined by the time at which melatonin levels reached 50% of the peak nocturnal values. Melatonin levels were measured during bright and dim phototherapy in the Seasonal Affective Disorder patients and during dim phototherapy in 5 controls. Morning light exposure advanced the time of onset of melatonin secretion and to a phase position similar to that of the normal controls while evening light delayed it compared to baseline levels. One interesting method to measure differences in sensitivity to light in different patients, is to investigate the suppressive effect of light on melatonin secretion from the pineal gland. The influence of melatonin on sexual behavior and breeding in different species inspired the pioneer investigations on melatonin levels in different phases of the menstrual cycle. In summary, melatonin levels in humans are influenced by genetic as well as environmental factors.
