Low melatonin secretion associates with albedo in circumpolar environments

Abstract

To clarify how seasonal changes of environmental luminosity affect melatonin secretion in human beings, nocturnal urine samples were collected every 2 months throughout the year from 20 outdoor workers living in an area, for which exact data on solar irradiance and temperature were available (67-68 degrees N). Melatonin secretion rates, determined by melatonin-specific radioimmunoassays, were found to be twice as high in December as in April, i.e. 0.88 +/- 0.16 nmol/12 hr (mean +/- S.E.) versus 0.43 +/- 0.09 nmol/12 hr, P < 0.05. Regression analyses showed that melatonin values correlated best inversely with solar irradiance reflected off the ground (albedo). A 10-fold increase in albedo for 1-4 weeks prior to the date of urine sampling was associated with a ca. 50% reduction in melatonin secretion. Association with global irradiance was weaker and did not correlate with temperature. Thus, light reaching the eyes via snow or other reflecting surfaces appears to be most effective. These results assist us in understanding some of the mechanisms involved in certain biological phenomena that exhibit seasonal variations as, for instance, reproduction and self-destructive behavior.

Seasonal Changes in Serum Total Thyroxine, Free Thyroxine, and Canine Thyroid-Stimulating Hormone in Clinically Healthy Beagles in Hokkaido.

“…The above tests make it possible to examine previously undetectable minute physiological changes in thyroid function. Many studies have reported that thyroid function in humans adapts to environmental conditions such as ambient temperature, photoperiod, and season, but the results have not been consistent because more than one parameter may mediate thyroid function [2][3][4]10]. Although similar findings were reported in rat [11,13], sheep [15], and dog [12], the only hormones measured were serum total thyroxine (tT 4 ), triiodothyronine (tT 3 ), and TSH.…”

“…Nishi et al [4] reported that TSH and fT 3 levels were higher during winter than in summer, but no such change was observed with fT 4 levels in healthy subjects in Sapporo Japan. Leppaluto et al of Finland [3] reported that fT 3 levels were lower in February than in August and TSH levels were higher in December than in other months, and that fT 4 , tT 4 , and tT 3 levels were unchanged. So that the results vary geographically, and the influences of ambient temperature and photoperiod are thought to account for these regional differences.…”

Association of melatonin secretion with seasonal luminosity in human subjects

“…We observed that melatonin secretion was twice as high in December as in April (0.88 nmol/12h vs. 0.43 nmol/12h), indicating the presence of a distinct seasonal rhythm in human subjects as well (11). Why this rhythm was not evident in Antarctica (12), was perhaps due to the fact REVIEW that during the Antarctic winter the subjects could spend outdoors only less than one hour in a day.…”

“…seals and reindeer stop their melatonin secretion almost totally in spring and summer (5). Several laboratories, including our own, have studied melatonin secretion in circumpolar areas and demonstrated that human subjects living at latitudes 60-70°N exhibit higher circulating melatonin levels in winter than in summer (6)(7)(8)(9)(10)(11). Interestingly, subjects overwintering in Antarctic bases did not exhibit seasonal changes in plasma melatonin nor in urinary melatonin metabolite levels, in spite of the seasonlong polar night.…”

“…Why this rhythm was not evident in Antarctica (12), was perhaps due to the fact REVIEW that during the Antarctic winter the subjects could spend outdoors only less than one hour in a day. Our subjects from northern Finland were exposed to an outdoor climate for 6-8 hrs daily throughout the year and exhibited a distinct seasonal rhythm (11).…”