MELATONIN RHYTHMS IN ARCTIC URBAN RESIDENTS

The 24-hr rhythm of salivary melatonin was measured in persons living in the city of Tromso (70 degrees N) at the following times of the yea r: in January at a day length of 2 hr of twilight, in June under conti nuous sunshine, and in March and September at about 12 hr light and 12 hr darkness. The hormone patterns varied widely between individuals, but, in general, they were consistent within most individuals-between the seasons. Highest peak values occurred in January when the mean lev el was also significantly higher than at any other time of year. The l owest mean levels occurred in June. Although individual rhythms were n ot always apparent, the mean patterns showed significantly elevated me latonin concentrations during the night at all seasons. The June melat onin peak was similar to that in March and September, but appeared to be phase-delayed with increased melatonin concentrations from midnight until 0900. It is assumed that the delayed melatonin peak in June may be associated with a tendency among people to shift their activity/re st rhythm and that the pineal sensitivity to light is reduced in the m orning in summer.