The seasonal interrelationship between melatonin, vasopressin, and serum osmolality in elderly subjects.

Plasma concentration of arginine vasopressin (AVP) and melatonin and serum osmolality were measured at noon and at midnight in individuals living in the northern hemisphere on March 22-23, June 13-14, September 26-27, and December 12-13 in 35 healthy volunteers (15 men and 20 women) aged 60-74 years. The nocturnal increase in melatonin was highest in the autumn and lowest in the winter in both sexes. The midnight serum osmolality level was lower in the autumn than in any other time of the year. In both the men and the women the AVP level was higher in winter than in any other season (P < 0.01 and P < 0.0001, respectively). In men, the AVP level was higher at noon than at midnight in 49% of the investigated 24 hr periods, at the same level in 15% and lower in 36% (NS). The corresponding figures for women were 55%, 25%, and 20%, respectively (P < 0.05). This study suggests a possible relationship between melatonin and serum osmolality.