Updated: Nov 5
Once the vibrant, fun packed summer comes to an end and we approach autumn and then winter, we can feel less vivacious and more lethargic than during the warmer months. This can affect mood and feelings of vitality; however, it is part of a normal cycle of life as we are intrinsically linked to the annual orbit of the Earth. Long understood by traditional practices, physiological studies now support the notion that
our biology is strongly influenced by the changing seasons, particularly through the effect they have on our hormones. This may at first seem a depressing thought, that feeling subdued during winter is normal. However, if we can understand this balance, work with the innate rhythm of the environment and nourish our bodies in the right way, it can help improve wellbeing during the darker, colder time of the year.
Evolution – why are our bodies and hormones affected by the seasons? Humans are a highly adaptive species who have long tried to override the inherent pattern of nature. Whether it be with artificial light, heat, or shelter, we consistently evolve to overcome the challenges of the natural world. Even so, we are still fundamentally affected by the seasons, physiologically, behaviourally, and emotionally.1
From an evolutionary perspective, all living creatures aim to immortalise themselves (or their genes) by reproducing. There is, however, a trade-off for the energy requirement of reproducing over that of survival. Therefore, the optimal time to give birth for the majority of creatures is the spring, when resources are plentiful. For humans, this means conceiving in the summer, not the winter when energy is scarcer. In order to cope with dynamic seasonal changes in the environment, seasonal behaviours, including reproduction, migration, hibernation, and moulting, have evolved as adaptive
strategies by which animals adjust their physiologies and behaviours based on the time of year. Animals detect changing seasons by a number of ways including thermal detection of ground temperatures, but most significantly by the changing daylight hours (or the photoperiod), known as the photoperiodism.1,2 Photoperiodism in humans is more unclear but there have certainly been associations of these seasonal changes with human physiology as well as behaviour, which include:
vitamin D metabolism
Many complex polygenic disorders, such as autoimmune, metabolic, cardiovascular, psychiatric, and infectious diseases, exhibit seasonal patterns of incidence. There is also an annual change in death rate, violent suicide, and mood. In addition, on a molecular level, we see changes in both white blood cells and adipose tissue.1 The mechanisms by which this happens it not fully understood but physiology shows that there are seasonal changes to hormonal regulation, which may complete the link.
Physiological changes to hormones As mentioned above, from an evolutionary perspective winter is not an optimal time for reproduction, therefore it may not be surprising that during this period there is a suppression of reproductive function via influence over hormonal regulation.
Reproductive hormones: Gonadotrophin Releasing Hormone (GnRH) – this is responsible for signalling from the hypothalamus to the pituitary to trigger the release of follicle stimulating hormone (FSH) and luteinising hormone (LH), essential for normal reproduction and the downward release of oestrogen and progesterone. It has been found in animals that during long summer days GnRH nerve terminals are in close proximity to the basal lamina, which supports GnRH signalling. Under short day (winter) conditions, the nerve terminals are encased by the endfeet of glial processes and separated from the basal lamina, thereby reducing the ability for GnRH to elicit an effect. Hence, there are morphological differences between reproductive hormone signalling during summer and winter, and reproductive signalling is reduced during winter months. This may not be so pronounced in humans but there is potential for a reduced reproductive function during this time.1 FSH – one epidemiological study found that sunshine increased FSH, which causes an egg to develop. This means women may not ovulate as frequently during the winter, so menstrual cycles may be longer and PMS symptoms worse. This is due to the fact that if ovulation rates are lower, less progesterone is produced and a high oestrogen to progesterone ratio can be a consequence of this.3 Oestrogen – as well as the indirect effect on oestrogen by changes to FSH, additional studies have identified that sunlight plays a role in directly affecting oestrogen. Research showed that receptors for oestrogen were more sensitive in the summer. Therefore, women are more fertile in the summer months as reduced oestrogen sensitivity can lead to problems ovulating. Again, this supports the natural benefit of reproducing in the summer.3,4 We’d love your comments on this article It’s easy, just post your questions, comments or feedback below This reduced sensitivity to oestrogen can have an additional effect on mood as it leads to reduced levels of serotonin. It can also exacerbate symptoms of the menopause, as symptoms are already related to a reduced production of oestrogen. This can all lead to feelings of reduced vitality during the winter. Testosterone – other studies have also shown that men produce less testosterone in winter, leading to reduced libido and sperm production and therefore reduced fertility.5,6
Effect of sunshine on hormones (and neurotransmitters!) Seasonal affective disorder (SAD) – SAD patients usually suffer from depressed mood, disrupted circadian rhythm, social withdrawal, and changes in appetite and body weight during the winter. Genetic factors, latitudes, and photoperiods are associated with the prevalence of SAD, and a photoperiodic mechanism similar to the one underlying seasonal reproduction has been suggested to be responsible for this syndrome.7,8 Serotonin and dopamine – exposure to sunlight is known to be essential for the production of serotonin, the feel-good neurotransmitter. Studies have confirmed that with shorter days and reduced sunlight exposure during winter, levels of both serotonin and dopamine (also important for mood) are depleted. This leads to the hypothesis that this variation is responsible, at least in part, for SAD.7,8 Vitamin D – although considered a vitamin, vitamin D possesses hormone like properties and is endogenously produced by the exposure of the skin to UV radiation (or sunshine). Vitamin D elicits multiple effects on the body including supporting bone health, immunity and mood. It has also been shown that vitamin D influences many other hormones including thyroid, insulin, oestrogen, and testosterone, as well as the neurotransmitters serotonin and dopamine.9 During winter months, the limited exposure of the skin to sunlight, reduces our ability to endogenously produce vitamin D and therefore it is important to maintain adequate intake of vitamin D during this time.10
Thyroid hormone The pineal gland, which produces melatonin from serotonin when light levels drop, can recognise photoperiod (duration of day light). It has been shown that during exposure to daylight the pineal gland may also influence thyroid hormone production. It appears that pineal melatonin may inhibit thyroid stimulating hormone (TSH) production. Therefore, shorter days or longer nights, can reduce thyroid function.1 However, evidence is contradictory as TSH appears to increase during colder weather.It is thought to stimulate heat production to maintain body temperature when weather is cooler. A small increase in free T4, Free T3 and T3 can also be seen, demonstrating enhanced thyroid hormone activity. There is also evidence that this is due to reduced temperatures as well as sunlight.11 This potentially means people are misdiagnosed as suffering from hypothyroidism, which is characterised by higher-than-normal levels of TSH, when actually their hormone levels are simply a response to the outside temperature. What can be seen is that thyroid function is altered, and it may be up or down regulated and therefore it is important to consider thyroid function particularly if wellbeing, vitality and reproductive capacity are reduced.
Traditional Chinese Medicine (TCM) – five elements and seasons TCM has long understood the power the seasons have over constitution. It describes this influence within the context of the five elements. Each element (wood, earth, fire, metal, and water) corresponds to a season, each of which has its own traits, both beneficial and challenging. If we are able to balance these elements, it can lead to a smoother transition throughout the seasons and be supportive for health.12 At the heart of TCM philosophy is the balance of yin and yang. To put it very basically, yin describes cooling and descending energy and looking inward, and yang describes heat, rising energy projecting outward. It is essential that these two opposing forces are in balance and are continually cycling through from one to the other. Often excess yang can be soothed with more yin practices and vice versa. The five elements is a complex principle, which is very briefly depicted below, and it should be noted that this is a basic description: Wood – Spring: time of new life and rising energy (yang). Considered a time to support the liver and gall bladder with light cleansing foods, young plants and fresh greens. Fire – Summer: period of luxurious growth, expansion, lightness, and outward activity (extreme yang). Nourish the fire elements with brightly coloured summer fruits and vegetables. A lively time of fun, joy and outward energy. The fire element rules the heart and small intestine. Earth – Late summer: this is the interchange of all seasons. This is the point of transition from yang to yin, between expansive growth and inward cooler seasons. Foods that support the earth season should be harmonised and represent the centre and includesweet yellow or golden foods such as squash, carrot, cabbage, and sweet potatoes. It is considered the season of the spleen and pancreas, important for vital energy but also immunity – so interesting that nourishing foods are beta-carotene and vitamin C rich. Metal – Autumn: this is when the harvest comes in and to pull inward and gather together, on all levels. The focus should be on fuelling and stocking up but also to study and prepare for the stillness of winter. Nourishing foods become heartier and warmer but also more astringent. Cooking methods should be more involved to supply a greater energy to the cooler season, which means longer slower cooking. Autumn is a time to support the lungs, which are associated with grief and sadness, and it is often a time when respiratory infections prevail. Water – Winter: this is the end of all seasons and is an introspective time to reflect, contemplate and meditate. Energy is descending (yin) and it becomes more inward, so it is a time to become inward focused. Salty and bitter flavours support the cold season as they promote a sinking, centreing quality. The kidneys and bladder are the organs of winter and are seen as the root and foundation of the body.12
Using TCM to support during Autumn and Winter If we look at the principles of TCM, we may notice that we tend to behave in ways that support the five elements without necessarily thinking about it. e.g., fresher vegetables and salads in spring and summer and more warming wholesome foods in autumn and winter. If we can accept that winter is a time for rest, recuperation and reflection and nurture this, it may not feel so daunting to face the darker days and longer nights.12 Some components that may help to nourish during the cold months:
Choose slowly cooked warming foods, stews, curries and soups, and include sweet yellows and oranges in autumn
Choose astringent (bitter and contracting) foods such as green tea, broccoli, cranberries and grapes
Choose bitter and salty foods in winter such as chicory, burdock, cabbage, miso and seaweed
Use Autumn and winter as a time to look inward, practice meditation and take up study
Aim to rest and take things more slowly but continue to move and stay mobile. Choose more yin activities such a walking, yoga and stretching
Additional activities and interventions to support during winter As well as looking at the balancing practices of TCM, we can utilise nutrition and lifestyle interventions to support physiology as it alters throughout the seasons:
Ensure you get adequate exposure to sunlight/daylight during the winter. This helps to stimulate serotonin production, which in turn helps with melatonin production thereby supporting the sleep/wake cycle. It may also be possible to get a little vitamin D (although sun is usually too weak October to May)
Support thyroid health with nutrients including tyrosine, iodine, zinc and selenium (ingredients and cofactors for thyroid hormone production)
Support a healthy stress response with relaxation techniques, regular exercise, ensuring stable blood sugar (lean protein, healthy fats and fibre). You can use nutrients such as vitamin C, B5, B6, magnesium and zinc to help support the adrenal glands (responsible for our stress response) as well as adaptogenic herbs such as ashwagandha or rhodiola
Look after the gut and liver, which are responsible for excretion of waste hormones and therefore hormone regulation
Ensure optimal intake of all nutrients to bridge the nutrition gap and nourish the endocrine system, particularly reproductive and thyroid
Consider supplementing vitamin D3 as endogenous production is greatly reduced. It is now recommended that everyone should supplement 10ug per day
Above all, it is important to be kind to ourselves, nurture and nourish our bodies and those around us. We can look forward to the cold season accepting that we may have a change in hormonal regulation and feel less energetic in the knowledge that the year will cycle around again. If we focus inwardly in winter, we will be more vital in the spring. Key takeaways
From an evolutionary perspective it is beneficial to give birth in the plentiful spring months, therefore it is practical that those who live in temperate climates have reduced fertility in the winter, and optimal fertility in the summer.
Studies have shown that there is a drop in follicle stimulating hormone in the winter which can lead to reduced fertility and a longer cycle worsening symptoms of PMS. Also, oestrogen receptors were shown to be more sensitive in the summer leading to improved fertility in summer compared to winter
Men have lower testosterone production, therefore lower libido and reproductive function, in the winter
Reduced sunlight during winter leads to reduced production of the feel-good neurotransmitters serotonin and dopamine, which is thought to contribute to seasonal affective disorder (SAD)
Thyroid function is also thought to be affected; reduced daylight may inhibit thyroid hormone production, but the cold weather can stimulate it. However, it should be noted that changes to thyroid function may impact health and wellbeing and should be considered
Vitamin D production is also impaired in the absence of adequate sunlight and therefore supplementation should be considered, again reduced vitamin D can continue to SAD
In traditional Chinese medicine (TCM) winter is considered a time to rest and reflect and nourish the body with warming foods, such as soups, stews and curries. It is good to include sweet yellows and oranges in autumn as well as astringent (bitter and contracting) foods such as green tea, broccoli, cranberries and grapes. Choose bitter and salty foods in winter such as chicory, burdock, cabbage, miso and seaweed
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Bright Light Exposure Increases Male Hormone — ScienceDaily. Accessed September 13, 2022.
Smith RP, Coward RM, Kovac JR, Lipshultz LI. The evidence for seasonal variations of testosterone in men. Maturitas. 2013;74(3):208-212. doi:10.1016/J.MATURITAS.2012.12.003
Lambert GW, Reid C, Kaye DM, Jennings GL, Esler MD. Effect of sunlight and season on serotonin turnover in the brain. Lancet. 2002;360(9348):1840-1842. doi:10.1016/S0140-6736(02)11737-5
Praschak-Rieder N, Willeit M. Imaging of seasonal affective disorder and seasonality effects on serotonin and dopamine function in the human brain. Curr Top Behav Neurosci. 2012;11:149-167. doi:10.1007/7854_2011_174
Vitamin D and healthy hormone balance | Cytoplan blog. Accessed September 13, 2022.
Berridge MJ. Vitamin D and Depression: Cellular and Regulatory Mechanisms. Barker EL, ed. Pharmacol Rev. 2017;69(2):80-92. doi:10.1124/pr.116.013227
Kuzmenko N v., Tsyrlin VA, Pliss MG, Galagudza MM. Seasonal variations in levels of human thyroid-stimulating hormone and thyroid hormones: a meta-analysis. Chronobiol Int. 2021;38(3):301-317. doi:10.1080/07420528.2020.1865394
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