Many individuals experience a connection between their sleep patterns and seasonal changes, with those in temperate zones feeling more lethargic in winter and more active in summer. Even tropical inhabitants can experience seasonal impacts. This behavioral adjustment to environmental cues stems from our evolutionary adaptation to seasons.

Read More

A recent study involving approximately 3,000 US medical interns, who wore health trackers for a year, indicates that this ancient adaptation also affects how individuals manage modern factors such as shiftwork and jet lag. It identified notable individual differences linked to variations in a specific gene called SLC20A2.

The interns generally had higher daily step counts and longer wake times in summer compared to winter. However, some participants exhibited minimal seasonal variation, while a few even demonstrated opposing trends. Although most were more active in summer, certain individuals rested more during that season.

Researchers utilized heart-rate data from the trackers to determine each intern's internal time—essentially what time feels like to them—based on their circadian rhythms, or body clocks. This internal timing influences various physiological factors, including body temperature and hormone levels. The study compared this information with participants' activity patterns to assess disruptions caused by night shifts.

Interns who exhibited the greatest differences in step counts between seasons experienced significant sleep-wake cycle disruptions following winter night shifts, but not after summer night shifts.

The study also explored the relationship between these findings and the SLC20A2 gene, previously linked to seasonal behavior in mice. This gene codes for a protein that facilitates ion exchange in cells, particularly affecting neurons in the brain and their electrical signaling.

Researchers identified thousands of variations in the SLC20A2 gene among participants, focusing on five specific differences known as single nucleotide polymorphisms (SNPs). They examined how these SNP combinations impacted individuals' behaviors across seasons. Through mathematical modeling, they demonstrated that certain genotypes affected circadian rhythms, physical activity, and adaptability to winter shiftwork.

Circadian rhythms are influenced by seasonal variations in daylight (photoperiod), crucial for the survival of various species. Humans, notably males, also show seasonal fluctuations in reproductive hormones, with testosterone levels peaking in spring and summer, despite not reproducing seasonally.

Biologists Colin Pittendrigh and Serge Daan proposed a model nearly 50 years ago suggesting that human circadian rhythms are regulated by two internal clocks responding to morning and evening light. This framework continues to explain how living beings adapt to seasonal changes in day length.

Light signals processed by the eyes are communicated to the brain's suprachiasmatic nuclei (SCN), which coordinate the response to varying day lengths. Research indicates that SCNs synchronize during shorter winter days but operate out of phase during summer's extended daylight.

Individual differences in light exposure, both natural and artificial, contribute to varied responses to seasonal changes, shiftwork, and jet lag. Night shift work is often associated with health issues such as weight gain and poor sleep quality. Understanding the biological underpinnings of adaptation to shiftwork may inform personalized health strategies for shift workers and those dealing with jet lag or seasonal adjustments.