Why the End of Summer Hits Your Brain Harder Than Winter Itself

In late September, long before any real cold arrives, something starts to shift inside the brain. And according to a recent scientific review, it isn’t the darkness that weighs the heaviest, but the speed at which it arrives.

That’s the central idea of one of the most comprehensive scientific reviews on the subject, published in 2023 in Translational Psychiatry by researchers Rui Zhang and Nora D. Volkow, the latter the director of the U.S. National Institute on Drug Abuse.

A year isn’t two seasons, it’s a curve

The usual intuition is to think of summer and winter as two blocks: lots of light, little light. Zhang and Volkow propose looking at something different: not the amount of light at a given moment, but the speed at which that amount changes day to day.

That speed isn’t constant across the year. It has two exact peaks:

• The shortening of the day is fastest around the autumn equinox, in late September.
• The lengthening of the day is fastest around the spring equinox, in March.

The body doesn’t just have to adapt to “less light” in autumn. It has to adapt to the moment when light changes fastest all year, and that moment arrives right as summer ends, not when winter truly begins.

What happens to a handful of neurons in October

One of the findings Zhang and Volkow cite is, at the very least, curious. In the suprachiasmatic nucleus, the brain structure that acts as the body’s master clock, there’s a specific group of neurons that produce vasopressin.

The finding comes from Dutch neuroanatomists Michel Hofman and Dick Swaab, who examined human brains donated for research from people who had died in different months of the year. In their 1992 and 1993 studies, the volume and number of these neurons peaked in early autumn, around October, and bottomed out in midsummer. Not in December, with the shortest days of the year, but when the light is changing fastest.

It’s a finding based on postmortem tissue and modest samples, so it shouldn’t be stretched too far, but it points to something bigger: the brain seems to react more to the environment’s rate of change than to a fixed state of darkness.

Neurotransmitters don’t all shift at the same pace

The same review describes how different neurotransmitter systems respond to the autumn transition unevenly, sometimes even in contradictory ways.

Serotonin. Several cited studies show its hypothalamic levels are lower in winter. PET neuroimaging studies also show that the serotonin transporter (SERT) tends to have greater availability in autumn and winter in the healthy population.

In people with seasonal affective disorder, that transporter becomes even more upregulated than expected in winter, which the authors interpret as a failure of the compensation mechanism that works fine in the rest of the population.

Dopamine. The findings here are less clean. Postmortem studies find lower dopaminergic activity in winter, while other neuroimaging studies find the opposite depending on which part of the circuit is measured.

Zhang and Volkow propose that melatonin, more prolonged during the darker months, would inhibit postsynaptic dopamine signaling while at the same time protecting the integrity of presynaptic dopaminergic neurons.

MAO-A. This enzyme, responsible for breaking down several neurotransmitters, naturally declines between winter and spring in healthy people.

In people with seasonal affective disorder, that seasonal decline simply doesn’t happen with the same intensity, something bright-light therapy partially reverses in just three weeks, according to the cited studies.

Why it doesn’t hit everyone the same way

This is where the review gets even more interesting: sensitivity to this mechanism varies enormously from person to person.

• Women show up to 1.5 times more risk of season-linked mood swings than men, according to the studies compiled.
• People with an evening chronotype (who perform and get going later in the day) report greater seasonal sensitivity, regardless of the latitude they live at or how much artificial light they use.
• As for the circadian rhythm’s own light sensitivity, a study by Andrew Phillips and colleagues (2019), which measured how much evening light suppressed melatonin in healthy adults, found a difference of more than 50-fold between the least and most sensitive person.
• Specific genetic variants, like the short allele of the 5-HTTLPR gene or a variant of the melanopsin gene (the photopigment that tells the biological clock how much light there is), show up associated with greater seasonal vulnerability in the studies reviewed.

None of these factors act alone. They combine, which is why two people can experience the same September in completely different ways.

The accidental experiment of living without electricity

One detail the review mentions in passing is especially striking: Amish communities, who use almost no electric light at night, show a much lower prevalence of seasonal affective disorder than the nearby Maryland population they were compared against.

The hypothesis on the table isn’t that they live somewhere sunnier, but that their exposure to natural light and natural darkness stays far more in sync with the sun’s actual cycle, without the interference of artificial light that, in modern urban life, blurs much of that seasonal signal.

When the metaphor helps, even if it isn’t science

None of this has any direct connection to Wintering (2020), the book by British writer Katherine May that popularized the idea of treating difficult periods as a natural resting season, not a personal failure.

It’s worth saying clearly: it’s a personal, popular-nonfiction book, not a scientific study, and none of its claims rest on the mechanisms described above.

But the very intuition running through the book (that certain stretches of the year call for less striving and more withdrawal) lines up curiously well with what these studies show: there’s a real biological reason, measurable in specific neurons and neurotransmitters, why the stretch from late August to October isn’t just any month for the nervous system.

What’s left once August ends

What science describes here isn’t a life sentence or a date to worry about. It’s, according to Zhang and Volkow, an adjustment process that most healthy people complete without difficulty, thanks to their neurotransmitters recalibrating along with the season itself.

That vague tiredness of the first weeks of autumn even has a popular name in Spain, astenia otoñal (autumn fatigue), which is not a clinical diagnosis; what is known about it is covered in what autumn fatigue is.

The detail worth keeping is this: if something feels different in your mood, sleep, or energy during the weeks after summer, it isn’t necessarily the cold, or the return to routine, or a coincidence. It is, to a large extent, the speed at which the planet itself is asking the brain to change pace.

If that adjustment hits you especially hard, the stress coping quiz can help you spot your own default pattern for handling change.

Sources

• Zhang, R., & Volkow, N. D. (2023). Seasonality of brain function: role in psychiatric disorders. Translational Psychiatry, 13, 65. https://doi.org/10.1038/s41398-023-02365-x
• Meesters, Y., & Gordijn, M. C. M. (2016). Seasonal affective disorder, winter type: current insights and treatment options. Psychology Research and Behavior Management, 9, 317-327.
• Hofman, M. A., & Swaab, D. F. (1992). Seasonal changes in the suprachiasmatic nucleus of man. Neuroscience Letters, 139(2), 257-260.
• Hofman, M. A., & Swaab, D. F. (1993). Diurnal and seasonal rhythms of neuronal activity in the suprachiasmatic nucleus of humans. Journal of Biological Rhythms, 8(4), 283-295.
• Levitan, R. D. (2007). The chronobiology and neurobiology of winter seasonal affective disorder. Dialogues in Clinical Neuroscience, 9(3), 315-324.
• Phillips, A. J. K., Vidafar, P., Burns, A. C., McGlashan, E. M., Anderson, C., Rajaratnam, S. M. W., Lockley, S. W., & Cain, S. W. (2019). High sensitivity and interindividual variability in the response of the human circadian system to evening light. Proceedings of the National Academy of Sciences, 116(24), 12019-12024. doi:10.1073/pnas.1901824116
• May, K. (2020). Wintering: The Power of Rest and Retreat in Difficult Times. Riverhead Books. (Cited as a cultural and literary reference, not a scientific source.)