Melatonin rhythms in Arctic urban residents

Area of Science:

• Chronobiology
• Human Physiology
• Arctic Research

Background:

• The human circadian system is influenced by environmental light cycles.
• Melatonin, a hormone regulating sleep-wake cycles, is sensitive to light exposure.
• High-latitude regions present unique challenges due to extreme seasonal variations in daylight.

Purpose of the Study:

• To investigate the 24-hour rhythm of salivary melatonin in individuals living at 70 degrees North.
• To assess how seasonal changes in day length (polar night and midnight sun) affect melatonin secretion patterns.
• To explore potential adaptations in circadian rhythms to extreme light conditions.

Main Methods:

• Salivary melatonin levels were measured over a 24-hour period in Tromsø, Norway.
• Measurements were taken during different seasons: January (minimal daylight), June (continuous daylight), and March/September (approx. 12-hour light/dark cycles).
• Individual hormone patterns and mean seasonal patterns were analyzed.

Main Results:

• Significant individual variation in melatonin patterns was observed, yet consistency within individuals across seasons was noted.
• Highest mean melatonin peak values and overall levels occurred in January (polar night).
• Lowest mean melatonin levels were recorded in June (midnight sun), with a phase-delayed peak observed from midnight to 0900.

Conclusions:

• Melatonin secretion exhibits seasonal variation in response to extreme light conditions at high latitudes.
• The observed phase delay in June suggests potential adjustments in activity/rest rhythms and reduced morning pineal sensitivity to light during Arctic summer.
• Individual circadian rhythms may adapt to prolonged periods of darkness or light, though consistency within individuals is generally maintained.