When Sunshine Becomes the Problem

The neuroscience of summer depression, and why your brain doesn't care that everyone else is thriving.

There’s a version of seasonal depression that arrives in June, peaks in August, and researchers have spent approximately forty years not studying why.

During my fellowship I occasionally saw clients in May presenting with what looked like new-onset depression. We’d get into the assessment and find nothing. No life event. No trauma. No obvious history. Just…the weather got nicer and something in them went very wrong.

My supervisor, without missing a beat, said summer SAD. Like it was the most obvious thing in the world.

It wasn’t obvious to me. Depression is supposed to be a winter sport. Everyone knows this. The sky turns the color of a dentist’s waiting room, vitamin D evaporates, and you spend most of your time canceling plans. Summer is supposed to be the antidote. Beaches. Grilling. People wearing crocs that have no business being worn in public. There is a universal cultural agreement that we should all be thriving.

Except for about 10 percent of people with seasonal affective disorder, summer is when everything goes to hell.

Lana Del Rey called it summertime sadness. Clinicians call it summer-pattern SAD. Your friends call it “I don’t know what’s wrong with me, it’s beautiful outside.”

Same thing.

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This article has been abridged, because, apparently, I talk to much for Substack. The full version has more science, more footnotes, and the part where summer is statistically making people violent. Read it for free here.

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The Depressed Minority

Winter SAD gets all the press because it affects about 5 percent of U.S. adults and we’ve built an entire cottage industry around $200 light therapy boxes. Summer SAD affects roughly 0.5 to 1 percent of the population, which sounds small until you realize that’s hundreds of thousands of people experiencing clinical depression while everyone else is posting Instagram photos with #sandlife!! (Kyeongmin, 2025).

The symptoms aren’t even the same. Winter SAD gives you the sleepies and the hungries. You want to burrow into a duvet and emerge in April holding a baguette. Summer SAD gives you insomnia, irritability, anxiety, agitation, and weight loss (Rosenthal et al., 1984). You don’t want to hibernate. You want to crawl out of your own skin and set fire to an ice cream truck.

Summer depression isn’t winter SAD backwards. The two have distinct neurobiological processes.

Your Brain’s Catastrophic Summer Failure

Let me introduce you to the serotonin transporter, or SERT.

SERT is a protein that sits on the surface of neurons and clears serotonin from the synaptic gap after it’s been released (Praschak-Rieder et al., 2008). Think of it as a cleanup crew. The more active SERT is, the faster serotonin gets pulled away, and the less available it is for mood regulation, sleep, and appetite.

In healthy people, SERT follows a predictable seasonal pattern. During fall and winter, SERT binding increases across the brain, so there’s less serotonin available. During spring and summer, SERT binding decreases which means more serotonin circulating. This is adaptive. Your brain is seasonally fine-tuning its own chemistry (Praschak-Rieder et al., 2008).

People with winter SAD overcorrect in the wrong direction they have too much SERT activity in winter, not enough serotonin, hence the depression (McMahon et al., 2016). Logic says summer SAD should show the opposite: SERT going haywire in summer. We don’t actually know if that’s true because nobody has studied it. Which is how a surprising amount of psychiatry still works.

What we do know is that summer heat destabilizes serotonin in ways winter darkness doesn’t.

Serotonin doesn’t just regulate mood. It also regulates body temperature. Your hypothalamus uses serotonin signaling to hold your core temperature at 98.6°F. Your hypothalamus is remarkably specific about this. When it gets hot outside, your body manages the heat partly by dumping serotonin. The collateral damage is your mood (Voronova, 2021; Morrow et al., 2020).

Healthy people heat-acclimate. Repeated exposure to heat that raises core temperature by at least 1.8°F for 60–100 minutes daily, over 5 to 14 days, and your body adapts resulting in lower resting temperature, better sweat efficiency, lower heart rate during exposure (Périard et al., 2015).

People with summer SAD appear to lack this capacity. Instead of acclimating, they deteriorate. June feels bad. July feels worse. August becomes unbearable.

The Sleep Disaster

Your body needs to drop its core temperature by 1 to 2 degrees Fahrenheit to fall asleep (Cajochen et al., 2003). This is non-negotiable. Your brain interprets being too warm as a sign that thermoregulation is failing and you should not fall asleep, because that’s how you die.

In summer, especially without air conditioning, your bedroom stays warm well into the night. Your body can’t cool down. Your melatonin doesn’t spike the way it should. And you lie awake at 2 AM contemplating the heat death of the universe while your partner snores peacefully beside you like some kind of thermally adapted pug.

It gets worse.

Your circadian clock doesn’t track sunset. It tracks dawn. In summer, dawn arrives much earlier, 5:30 AM instead of 7 AM, and your circadian clock responds by shifting your entire sleep-wake cycle earlier (Kantermann et al., 2007). Your biology schedules bedtime for 8:30 PM. But melatonin requires darkness, and at 8:30 PM in July the sun is still blazing. Meanwhile your work schedule hasn’t moved. Your friends aren’t having happy hour at 4 PM. Daylight saving time has pushed everything forward another hour.

You’re biologically ready for bed while the sun blazes and your phone buzzes with dinner invitations, and you can’t sleep because the evening light is blocking the melatonin your circadian system desperately wants to produce.

Chronic circadian disruption increases cortisol, tanks serotonin, and creates the exact neurochemical environment that causes depression (Wehr et al., 2001). Add progressive heat-induced serotonin depletion and you have a recipe for disaster, served with insomnia and garnished with rage.

What Actually Helps

Keep your environment cool. Air conditioning is not a luxury if you have summer SAD, it’s a medical intervention. If you can’t afford central AC, get a window unit. If you can’t afford a window unit, spend time in libraries, malls, cooling centers. You are one broken AC unit away from a mental health crisis. Act accordingly. Take cool showers before bed. Use cooling pillows. Do whatever drops your core temperature enough to trigger sleep (Cajochen et al., 2003).

Manage evening light. Your instinct to hide in your apartment with blackout curtains is clinically correct. Dim the lights after dinner. Avoid screens before bed — blue light suppresses melatonin harder than ambient light does (Gooley et al., 2011).

CBT for SAD works on two tracks: replacing catastrophic seasonal thinking (”something is fundamentally wrong with me”) with something accurate (”my thermoregulatory system has a seasonal failure and this is not a character flaw”), and behavioral activation — the clinical term for doing something small that isn’t laying in a dark room catastrophizing (Rohan et al., 2016). It won’t fix a broken hypothalamus, but it will stop your brain from making a biological problem worse.

SSRIs block SERT, which is the mechanism that’s likely misfiring. Sertraline and fluoxetine both have evidence for SAD. Bupropion is FDA-approved for preventing seasonal depression (Galima et al., 2020). Some people need medication only in summer. Both seasonal and year-round use are fine. Talk to a psychiatrist.

If you know summer is coming for you — and you do, because summer SAD is predictable — start treatment in spring. Don’t wait until you’re in crisis in July.

What This Is

Summer depression violates the social contract we have with seasons. If you’re depressed in February, people nod. If you’re depressed in July, they offer a smoothie and a personality critique.

But the physiology is not subtle. Heat stress, sleep disruption, and circadian misalignment change serotonergic function. This can look like anxiety, irritability, burnout, treatment resistance, anger issues. It is none of those diagnoses until you rule out the season.

Stop moralizing a thermoregulatory failure. Stop prescribing gratitude.

Sunshine isn’t universally therapeutic. Sometimes it’s just bright.

Recommended Reading

If you want to understand SAD and seasonal patterns:

Winter Blues by Norman Rosenthal. Rosenthal coined the term SAD. He is the researcher who started all of this. The book skews winter because that’s where his research began, but the foundational science is all here and he’s a readable clinician who doesn’t talk down to you. Start here if you want the origin story.

If you want to understand sleep and circadian biology:

The Circadian Code by Satchin Panda. Panda runs one of the leading circadian biology labs in the country and writes like someone who actually wants you to understand the material. The melatonin and sleep timing sections of this article live inside this book at greater length. Less alarming than Walker’s Why We Sleep, which is a recommendation in itself.

Why We Sleep by Matthew Walker. More famous than Panda and occasionally accused of overstating his findings, which is fair. But for understanding why your sleep falls apart in summer, it’s the most accessible entry point available. Read it, just don’t spiral.

If you want to understand depression:

The Noonday Demon by Andrew Solomon Not SAD-specific. The most honest and comprehensive book on depression a non-clinician has ever written. If this article opened a door you weren’t expecting, this is where you go next. Fair warning: it’s long and it earns the word “heavy.”

If you want to understand heat, behavior, and what’s coming:

Behave by Robert Sapolsky. The heat, serotonin, and violence section of this article is a footnote in Sapolsky’s universe. Seven hundred pages on why humans do terrible things. Somehow completely riveting. Sapolsky is funny, which helps when the subject matter is human beings at their worst.

Heat Wave by Eric Klinenberg. Klinenberg’s forensic account of the 1995 Chicago heat wave that killed over 700 people in a week. Part social autopsy, part indictment of how seriously we fail to treat heat as a public health crisis. The chapters on who died and why will make the air conditioning section of this article feel less like advice and more like urgency.

The Uninhabitable Earth by David Wallace-Wells. If the heat-serotonin-violence section of this article made you uncomfortable, this book will finish the job. Wallace-Wells makes the case that rising global temperatures aren’t just an environmental problem, they’re a psychiatric one. Not a light read. Not designed to be.

I’m an Amazon Associate and earn from qualifying purchases. This supports the deep dives. If you choose to buy, I appreciate it. But your local library got there first, so I’d start there.

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Works Cited

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Galima, S. V., Vogel, S. R., & Kowalski, A. W. (2020). Seasonal affective disorder: Common questions and answers. American Family Physician, 102(11), 668–672

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