The last week or so I felt absolutely drained of energy and wanted to sleep a lot more than usual. I found myself thinking, “I shouldn’t be sleeping so much," “I should be more social,” “I should be exercising more."... “I should be doing more, more, more.” But the truth is, I just don't feel like it!
“Maybe I should get a blood test? (Health-anxiety brain circuit getting tested!)
Almost everyone I’ve shared this with, it turns out, has also been feeling extra-fatigued this week. So I started wondering (as is my habit) about the connection between human biology and seasonality. Surely, if there are daily cycles controlled by biophysics (circadian rhythms) there must also be annual rhythms? Here’s what I found out.
Why do we have seasons?
The Earth is tilted 23.5 degrees on its axis, which affects the distribution of the sun's energy across the surface of the planet. As the Earth orbits the sun every 365 ¼ days, the axis is always pointing in the same direction into space, with the North Pole toward Polaris, the North Star (NOAA, 2022). If you don’t live in an equatorial zone, this tilt controls average length of daylight and daily temperature range and variations where you are on the planet. All organisms that are not equatorial have evolved to adapt to this seasonality - that’s why trees lose their leaves in autumn/fall, right, and why mammals hibernate? So, I wondered, aren't we really the same?
We're organisms in this solar system, evolving in the same spaces as those trees.
It seems that in northern latitudes our very ancient ancestors did indeed engage in a form of winter hibernation and although humans no longer hibernate, there is some evidence that we do retain the genetic hardware to do so. Hibernation allows creatures that do it to survive in a near-frozen state for extended periods of time. If humans could hibernate, the theory goes, there would be great health benefits - it would boost longevity and enable us to combat modern diseases like Alzheimers, heart disease and cardiac arrest (Wehr, 1996).
On the other hand, evidence from an anthropological study of early human fossilised remains suggests that some ancestors, during the last ice-age, did indeed hibernate although evidence showed they were not so healthy (kidney disease, rickets and mineral and bone disorders), presumably due to lack of daylight. The two studies taken together suggest there may be a few things we can tweak about our lifestyles to get some health benefits, whilst at the same time avoiding the rickets.
A new scientific study (2023) carried out on material taken from nearly 1000 human cadavers finds evidence of seasonality in human gene expression, similar to those observed in birds and other mammals. With a bit of reflection it’s a no-brainer, isn’t it? I mean it should be obvious.
The study discovered substantial seasonal remodelling of the cytoarchitecture of certain areas of the human brain and noted an increase in "astrocytes" in autumn and winter. Astrocytes perform metabolic (the complex chemical process that combines calories and oxygen to create and release energy), structural (maintaining the structure of important tissues such as the blood brain barrier), homeostatic (body temperature regulation), and neuroprotective tasks, such as clearing excess neurotransmitters, stabilising and regulating the blood-brain barrier, and promoting synapse formation.
The study also found a significant enrichment of immune related genetic markers during winter/fall, whilst mRNA levels of pituitary hormones peaked in summer. Genetic markers were found, as in birds and other mammals, that relate to seasonality in mating, breeding, foraging and hibernation.
A further study from 2021 analysed data from millions of blood samples taken in Israel and was able to demonstrate the strong seasonality of circulating hormones.
So, the changing seasons change the way our genes are expressed and create significant changes in the structure and functions of our organs and nervous system and influence levels of circulating hormones in our blood, in such a way as to significantly change our biology from season to season.
In other words, the changing seasons naturally affect our biology and this includes our energy and activity levels, food consumption propensity and even sexual behaviour (based on the best times of year for giving birth).
In a study of three contemporary pre-industrial societies, Yetish et al (2015) showed that modern humans living without the intervention of electric light sleep an average of one hour longer each night during winter months than in summer months, again suggesting that more sleep in the winter months is normal for humans living seasonally.
Is it even possible that, when we don’t adapt, move and change along with the seasons, it affects our wellbeing negatively?
Here are the physical symptoms of Seasonal Affective Disorder according to the NHS website:
feeling lethargic (lacking in energy) and sleepy during the day
sleeping for longer than normal and finding it hard to get up in the morning
craving carbohydrates and gaining weight
difficulty concentrating
decreased sex drive
Does it sound like there's link there? In the light of the studies I've mentioned these symptoms of the changing season sound completely normal and natural for humans living in these higher latitudes. Of course, these symptoms do make it difficult to function at the level often demanded by work, which remains the same throughout the year. Finding it difficult to cope can lead to negative thoughts and depressed feelings about self-worth and ability to manage in life etc. Treatment may be necessary to enable a person to function as required.
Of course, the main conclusion of the scientists in the 2023 study was that their findings indicated better and ever more effective ways of delivering drugs and the opportunities to make new drugs.
Drugs are sometimes helpful and sometimes necessary, but to my way of thinking these studies are highly significant in terms of pointing us towards better, more sustainable, ways of living and organising our societies in a way that could maximise health and wellbeing outcomes. It also suggests a useful social message to give people about their own biology, about what is, “normal,” and about how we can live well through the seasons. Human biology evolved to work with the cycles of the planet, not with the cycles of modern industrial societies.
Is it just normal to need more rest and to go slower and be more mindful at this time of year, as nature begins to shut down for the winter?
Maybe I could tone down my exercise routine and do something more calming and restorative.
Is it just normal to have the impulse to consume more food and more calories, as we should be preparing for (what was once) a period of rest and scarcer food? Maybe I could be extra mindful about my consumption and ensure that any extra food I do eat is mostly fruit, vegetables, nuts or seeds. My guts did not evolve for the processing of pies and cakes - but it is true that I am biologically drawn to this over-processed, highly calorific, under-nourishing fodder.
Is it just normal to want to spend more time in bed when everything outside nature is shutting down? For me, I’m going to try out allowing myself close down a bit, letting it be OK to not be full-on 100% (or even 50%). I’m going to light candles in the dark, turn Netflix off, and go to bed stupidly early. I’ll go for walks to get my daylight dose in, take a D3 supplement and watch what nature is doing at this time. I’ll try it for a few weeks before I give in to that health anxiety brain circuit of mine.
What do you think about this?
If you are affected by seasonal affective disorder and/or depression, have sought the advice of your doctor, and would like to try some TEAM CBT treatment, send me a DM - I have some great tools that can help.
References
Article Source: Day-night and seasonal variation of human gene expression across tissues Wucher V, Sodaei R, Amador R, Irimia M, Guigó R (2023) Day-night and seasonal variation of human gene expression across tissues. PLOS Biology 21(2): e3001986. https://doi.org/10.1371/journal.pbio.3001986