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Chronobiology – How the Cold Seasons Influence Our Biorhythms

16.12.2016

The colder seasons of the year are not only freezing - the reduced light intensity also deeply influences our physical functions, our mood and our behavior.

Image source: Walter Schießwohl

We wake up when it’s still dark… Through the day we try to catch a fleeting glimpse of sunlight and in the evening, we leave work at dusk...

Autumn and winter are the dark seasons. The lack of light influences our mood, our vital parameters and our internal biorhythms.

An internal clock

Like all living beings, from bacteria through plants and animals, humans follow internal rhythms and cycles throughout their whole lives. We have a central clock which coordinates key bodily functions including our sleep-wake cycle, our heart rate and the body’s temperature. Cycles of activity or hormone release are also influenced by our internal periodic control system. Although most biorhythms follow daily cycles, shorter and longer intervals also occur. Some rhythms for example are based upon lunar phases or, as described here in detail, upon the changing seasons. The field of science which studies these rhythms is called chronobiology.

Biorhythms are internal responses to environmental stimuli. The major factor in this process is sun light which is primarily responsible for regulating the body’s internal clock. But how does this work?  

It is a complex interaction between the brain and the individual cells within our body. Special receptors in our eyes respond to light intensity and transfer this information via the optic nerve to specific nerve cells within the brain’s core. This small and central part of the brain is called the suprachiasmatic nucleus and it performs the role of the body’s master clock. It communicates with other parts of the brain and regulates light dependent hormone release. This, in turn, stimulates millions of organ and tissue specific “clocks” within individual cells.

Darkness triggers…

How does winter influence our biorhythms? In a nutshell, it’s because of the absence of sunlight during this time of the year. The most extreme situation is found around the polar areas where the sun does not appear at all for 6 months. However, even in other regions which are far enough from the equator to have seasons, the days get shorter from autumn through to winter.

The low light levels cause a higher release of melatonin, the so-called sleeping hormone. Additionally, less activity hormones are produced because melatonin also decreases action within the thyroid gland. Therefore, people often feel more tired and listless in autumn and winter. Recent studies also suggest that increased melanin can affect levels of aggression through stimulating the production of the precursors of the sex hormones androgen and estrogen.

Body functions adapt to winter

Every year many people suffer from seasonal affective disorder (SAD) or the “winter-blues”. One reason is the low concentration of the light-dependent happiness hormone, serotonin, during the dark months.

However, not everything functions badly during the colder seasons. Studies using mice have revealed, for example, that their immune systems actually become stronger with the shortening of the days in autumn. This could also apply to humans since many regulatory cycles are similar within different mammal species. Another quite surprising finding in chronobiology is that the performance of our brain is also seasonally dependent and that it actually functions more effectively in winter. In studies, participants had to solve cognitive challenges at different times of the year whilst scientists simultaneously analyzed the location and the intensity of brain activity. Although, the quality of solving the challenges was consistent for each participant throughout the whole year, brain activity, in contrast, changed with the seasons. In winter, smaller amounts of effort were required from the brain to achieve the same quality of results as in the summer. The brain simply functions more efficiently in winter.