According to Chinese medicine, the human body must conform to the four seasons' patterns and balance it with nature.
A review of 3.5 Million blood tests has shown a whole host of human hormones that fall into clear seasonal patterns, albeit with minor variations.
Pituitary hormones help control reproduction, metabolism, stress and lactation, and usually peak in late summer.
Peripheral organs under pituitary control, such as those that make sex hormones or thyroid hormones, also show seasonality. However, these hormones' levels do not peak in the summer but rise in the winter. Just as testosterone, oestradiol and progesterone peak in late winter or late spring.
This finding provides the most substantial evidence to date that humans have an internal seasonal biological clock that aligns physiological rhythms with the seasons.
The group of authors write, "There is a long history of research on the winter-spring peaks in hormone levels that combine human function and growth, and the seasonal pattern of hormones suggests that, like other animals, humans may have physiological peaks in basic biological functions (similar to the estrus period in animals)."
The mechanisms behind the seasonal biological clock remain unknown, but the authors suggest a natural year-long feedback loop between the pituitary gland and peripheral glands in the body.
Pituitary hormones regulated by daylight modulate other organs over the year, thus prompting them to adapt to seasonal changes.
As the paper points out, it is not too different from the phenomenon we see in other mammals, wherein some mammals fluctuations in certain hormones cause seasonal changes in animal reproduction, activity, growth, pigmentation or migration.
For example, mammals such as Arctic reindeer reduce levels of a hormone known as leptin in winter, lower their energy expenditure, reduce body temperature and inhibit reproduction.
Even primates closer to the equator show sensitivity to seasonal changes. For example, rhesus monkeys ovulate significantly more in the post-monsoon season, as it is more favourable for offspring to be born before the onset of the summer monsoon.
So far, the data on humans is mostly inadequate and does not cover all hormone types, but if our hormones do fluctuate with the seasons, even just a little, that would be important for health.
Hormone seasonality in medical records suggests circannual endocrine circuits
Avichai Tendler, Alon Bar, Netta Mendelsohn-Cohen, Omer Karin, Yael Korem Kohanim, Lior Maimon, Tomer Milo, Moriya Raz, Avi Mayo, Amos Tanay, Uri Alon
Proceedings of the National Academy of Sciences Feb 2021, 118 (7) e2003926118; DOI: 10.1073/pnas.2003926118
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