Spring snow lowers human melatonin

Abstract

Objectives. We set out to find out in which way seasonal changes of environmental luminosity could affect melatonin secretion in humans. Study design. For an entire year we collected every two months nocturnal urine samples from 20 male outdoor workers who lived and worked in an area of the circumpolar region from which exact data of solar irradiance and temperature were available. Methods. Melatonin secretion rates were assessed with our melatonin-specific radioimmunoassay. Results. Melatonin secretion was twice as high in December as in April, 0.88 ± 16 nmol/12 h (mean ± SE) vs. 0.43 ± 9 nmol/12 h, p < 0.05. Regression analyses showed that melatonin values best correlated inversely with solar irradiance reflected off the ground (the so-called “albedo”). A tenfold increase in albedo for 1–4 weeks before the urine samplings were collected was associated with a ca. 50% reduction in melatonin secretion. The association with global irradiance was weaker and none existed with temperature or other environmental variables. Discussion. Light reaching the eyes via the snow or other reflecting surfaces appears to be most effective. These results help us to understand some of the mechanisms involved in certain biological phenomena that exhibit seasonal variations such as reproductive and selfdestructive behaviours.

Key Words:

• urinary melatonin
• seasonal
• annual
• biorhythm
• solar irradiance
• visual perception