A review of millions of blood tests has shown a whole host of human hormones that fall into clear seasonal patterns, although these changes are small in magnitude.
Hormones from the pituitary gland, which help control reproduction, metabolism, stress and lactation, were mostly found to peak in late summer.
Peripheral organs under the control of the pituitary, like those that produce our sex hormones or the thyroid hormone, also showed seasonality. Instead of peaking in summer, however, these hormones hit their stride in winter.
Testosterone, estradiol, and progesterone, for instance, reached their pinnacle in late winter or spring.
The findings provide the strongest evidence to date that humans possess an internal seasonal clock, which somehow impacts our hormones in a way that lines up with the seasons.
"Together with a long history of studies on a winter−spring peak in human function and growth, the hormone seasonality indicates that, like other animals, humans may have a physiological peak season for basic biological functions," the authors write.
The underlying mechanism that drives this circannual clock is still unknown, but the authors suggest there is a natural, year-long feedback circuit at play between the pituitary gland and peripheral glands in the body.
The pituitary hormones, which are uniquely tuned to sunlight, could be feeding these other organs over the course of a year, allowing them to grow in functional mass in a way that aligns with the seasons.
"Thus, humans may show coordinated seasonal set-points with a winter−spring peak in the growth, stress, metabolism, and reproduction axes," the authors write.
As the paper mentions, it's not too different from what we find in other mammals, where fluctuations in certain hormones lead to seasonal changes in an animal's reproduction, activity, growth, pigmentation, or migration.
Mammals like arctic reindeer, for instance, show a decrease in a hormone called leptin when winter days become shortest, and this helps lower their energy consumption, decreasing their body temperature and inhibiting their ability to reproduce.
Even primates closer to the equator show sensitivity to subtle seasonal changes. For instance, Rhesus macaques ovulate significantly more during the post-monsoon season so that their offspring are born just before the monsoons hit in summer.
Whether or not human hormones also fluctuate with the seasons remains unclear.
Most datasets that have been analysed so far are not very large and do not cover all human hormones, which makes drawing conclusions very challenging. Studies have either examined only human sex hormones, or they have focused on stress and metabolic hormones. Results have also been quite varied and inconsistent.
Meanwhile, research on salivary cortisol levels – aka the stress hormone – finds there is some seasonal variability, and a big data study on the thyroid-stimulating hormone found higher levels of this hormone in summer and winter.
The new research is the largest of the lot and includes a massive dataset of Israeli health records covering 46 million person-years. It also analyses all human hormones.
Controlling for changes throughout a single day, the authors found humans do show seasonal patterns in their hormone levels, although not as strongly as other mammals.
The physiological effects of these hormonal shifts are still not clear, but some of the changes to the thyroid hormone, T3, and the stress hormone, cortisol, do align with previous findings.
For example, the thyroid hormone, which was found to peak in winter, has been tied to thermogeneration. The seasonal timing of cortisol, which was found to peak in February, also agrees with past studies spanning the northern and southern hemispheres.
The seasonal changes are small in magnitude, but as the authors point out, from a clinical perspective, "even a small systematic effect can cause misdiagnosis if the normal ranges are not adapted to the seasons, with associated costs of extra tests and treatment."
More studies on a similarly large scale and in various parts of the world will need to be done to verify the results further. But the findings suggest we are not so different from other mammals after all.
If our hormones really do ebb and flow with the seasons, even just a little bit, it could be important for our health that we know.
The study was published in PNAS.