Seasonal affective disorder
Based on Wikipedia: Seasonal affective disorder
Every winter, millions of people undergo a strange transformation. They sleep longer but feel more exhausted. They crave bread, pasta, and sweets with an almost primal urgency. Their social lives wither. The world seems to drain of color and meaning—not because anything in their lives has actually changed, but because the sun has retreated.
This is seasonal affective disorder, and it sits at a fascinating intersection of psychology, biology, and our ancient evolutionary past. It raises questions about what depression actually is, whether our modern indoor lives are fundamentally incompatible with our biology, and why some populations seem mysteriously immune to its effects.
More Than Just the Winter Blues
Let's be clear about what we're discussing. Seasonal affective disorder—often abbreviated as SAD, in what must be one of medicine's more on-the-nose acronyms—is not simply feeling a bit down when it's cold outside. It's a form of major depressive disorder, carrying all the weight that diagnosis implies: feelings of hopelessness and worthlessness, thoughts of suicide, withdrawal from friends and family, an inability to concentrate or make decisions, and a pervasive loss of interest in activities that once brought joy.
What makes it distinct is the pattern. People with SAD experience these symptoms at the same time each year, like clockwork tied to the seasons. When spring arrives, the depression lifts—sometimes vanishing so completely that the person returns to entirely normal mental health until the following autumn.
The winter variety is the most common and most studied. Its symptoms have a particular character: extreme fatigue, sleeping far more than usual yet never feeling rested, and intense carbohydrate cravings that lead to weight gain. Some people fall asleep within five minutes of lying down in the evening—a sign of profound exhaustion that would be unusual in typical depression, where insomnia often prevails.
But here's something that surprises many people: seasonal affective disorder isn't exclusively a winter phenomenon. A smaller but significant number of people experience the opposite pattern. Their depression arrives with the long days of summer, bringing insomnia instead of oversleeping, weight loss instead of gain, and an agitated anxiety rather than the sluggish withdrawal of winter SAD. The arrival of shorter autumn days brings relief.
The Geography of Sadness
If you suspected that seasonal depression has something to do with sunlight, the geographic data would seem to confirm your intuition. In Florida, where winter days remain relatively bright and long, about one and a half percent of the population experiences SAD. Travel north to Alaska, where winter means only a few hours of dim twilight, and that figure jumps to nearly ten percent.
The pattern holds across the globe. In northern Finland, at about sixty-four degrees latitude—where the sun barely rises above the horizon during the darkest months—researchers found SAD affecting almost one in ten people.
Yet this neat correlation has a troubling exception.
Iceland should be a disaster zone for seasonal depression. Situated just below the Arctic Circle, the country experiences winter days with only four hours of murky daylight. By all rights, Icelanders should be among the most affected people on Earth.
They're not. Studies of more than two thousand Icelanders found unexpectedly low rates of seasonal affective disorder. Even more striking, when researchers studied Canadians of Icelandic descent—people living under the same Canadian skies as their non-Icelandic neighbors—they too showed unusually low rates of SAD.
Something genetic seems to protect this population. One theory points to diet: Icelanders traditionally consume enormous quantities of fish, about ninety kilograms per person per year compared to roughly twenty-four kilograms in North America. Fish contains vitamin D and omega-three fatty acids, both of which have been implicated in mood regulation. Perhaps centuries of living in extreme latitudes naturally selected for people whose biochemistry could cope with minimal sunlight, or perhaps their diet compensates for what the sun cannot provide.
The Biology of Darkness
To understand what happens in the brain during seasonal depression, we need to talk about two molecules: serotonin and melatonin.
Serotonin is the neurotransmitter most associated with mood and well-being. It's the target of most modern antidepressants, which work by keeping more serotonin available in the brain. One theory of SAD proposes that reduced sunlight somehow depletes serotonin or interferes with its function.
The connection isn't straightforward, though. Mice that can't properly convert serotonin into related compounds do seem to display depression-like behavior, and antidepressants appear to work in part by affecting these conversion processes. But human studies have been less definitive. Some research supports the serotonin hypothesis; other studies dispute it.
Melatonin tells a more compelling story. This hormone is produced by a tiny structure in the brain called the pineal gland, and its production follows a simple rule: darkness triggers it, light suppresses it. Melatonin is the body's internal signal that night has arrived—time to wind down, sleep, and conserve energy.
The connection between your eyes and your pineal gland is remarkably direct. A dedicated neural pathway runs from the retina through a brain structure called the suprachiasmatic nucleus—the body's master clock—and on to the pineal gland. When light enters your eyes, even if you're not consciously aware of it, this pathway can suppress melatonin production.
During winter, with its long nights and dim days, people with SAD may produce melatonin on a shifted or extended schedule. Their internal clocks drift out of alignment with the external world. They're biologically signaled to sleep and conserve energy at times when modern life demands they be awake and productive.
This circadian rhythm disruption—the misalignment between internal biological time and external clock time—may be the core problem. It would explain why bright light therapy, which powerfully suppresses melatonin, helps so many people with SAD. The light isn't just making them feel cheerful; it's resetting their internal clocks.
An Evolutionary Perspective
Here's a question worth sitting with: is seasonal affective disorder actually a disorder at all?
Consider what happens to animals during winter. Food becomes scarce. Cold temperatures make survival more costly. Across species after species, behavior changes: activity decreases, eating patterns shift, social interaction diminishes. Hibernation is the extreme version, but even animals that don't hibernate typically slow down during the cold months. They conserve energy, eat when food is available, and sleep more.
The symptoms of winter SAD—hypersomnia, carbohydrate craving, social withdrawal, reduced activity—look remarkably like a human version of this ancient mammalian winter response. Perhaps what we call a disorder is actually an adaptive program, written into our genes across millions of years of evolution, that simply makes no sense in a modern world of electric lights, heated buildings, and year-round food supply.
From this perspective, the remarkable thing isn't that some people experience seasonal depression. It's that most people don't—that we've somehow overridden a behavioral pattern that served our ancestors well for millennia.
Shining Light on Treatment
The most distinctive treatment for seasonal affective disorder is light therapy, and its effectiveness offers further evidence that disrupted circadian rhythms lie at the heart of the condition.
A light therapy box is far more powerful than an ordinary lamp. While a typical living room might be lit to around three hundred lux—the unit used to measure light intensity—a therapeutic light box produces ten thousand lux, roughly equivalent to being outdoors on a bright overcast day. Patients sit near the box, eyes open but not staring directly at the light, for thirty to sixty minutes each day.
The results can be dramatic. Many patients notice improvement within a week, though the full benefit typically builds over several weeks. Studies consistently show light therapy working as well as antidepressant medications, with about two-thirds of patients experiencing significant relief.
The color of light matters. For years, researchers believed that blue light was most effective, since blue wavelengths most powerfully affect the circadian system. But more recent research suggests that green or broad-spectrum white light may work as well or better—and may be easier on the eyes for daily use.
An even gentler approach is dawn simulation. Instead of sitting in front of a bright light, patients use a device that gradually increases illumination in their bedroom before they wake, mimicking a natural sunrise. Some studies have found this even more effective than standard bright light therapy, perhaps because it aligns more naturally with the body's wake-up process.
There's an important caveat about light therapy: about nineteen percent of patients stop using it because they find it inconvenient. Sitting in front of a light box for an hour each morning requires restructuring one's routine. And while some people can use the time productively—reading, checking email, eating breakfast—others find it simply doesn't fit their lives.
The Medication Question
Standard antidepressants work for seasonal affective disorder, which makes sense given that SAD is, after all, a form of major depression. The selective serotonin reuptake inhibitors—fluoxetine (Prozac), sertraline (Zoloft), paroxetine (Paxil)—have all proven effective.
Direct comparison trials found fluoxetine and light therapy equally effective, both helping about two-thirds of patients. But they work differently over time. Light therapy produces faster initial improvement, often within the first week. Medications build more gradually but may be easier to maintain since they don't require daily sessions with a light box.
One medication deserves special mention: bupropion, sold as Wellbutrin, can actually prevent seasonal depression when started in the autumn before symptoms begin. Studies show it reduces the risk of winter depression by about twenty-five percent. This preventive approach makes sense for people with reliable, predictable seasonal patterns who know that every winter brings a depressive episode.
Vitamin D supplements represent a more controversial option. The logic is appealing: sunlight triggers vitamin D production in the skin, winter means less sunlight, therefore vitamin D levels drop, therefore supplementation should help. Some people do find vitamin D helpful, but clinical studies have been disappointingly inconsistent. Large trials haven't shown clear benefits, possibly because the relationship between vitamin D and mood is more complex than simple supplementation can address.
The Bipolar Complication
About one in five people with seasonal depression actually have bipolar disorder rather than straightforward major depression. This matters enormously for treatment.
Bipolar disorder involves alternating episodes of depression and mania or its milder form, hypomania. Mania brings elevated mood, decreased need for sleep, impulsive behavior, and sometimes grandiose thinking. Hypomania is similar but less extreme—a person might feel unusually energetic, productive, and confident without the severe impairment that characterizes full mania.
For people with bipolar disorder who experience seasonal patterns, the winter brings depression and summer may bring hypomania or mania. The danger is that treating winter depression with antidepressants alone can trigger a manic episode. Light therapy carries the same risk—it's essentially giving the brain a powerful stimulus that says "it's summer now," which for a bipolar brain might flip the switch too far in the opposite direction.
Proper diagnosis matters. People with bipolar disorder typically need mood stabilizers in addition to or instead of antidepressants. The seasonal pattern in bipolar disorder tends to associate with more severe illness overall: more depressive episodes, more rapid cycling between mood states, and higher rates of eating disorders.
A Milder Version
Not everyone who struggles with winter experiences the full syndrome. Subsyndromal seasonal affective disorder—sometimes abbreviated as sub-SAD or s-SAD—affects a much larger portion of the population, perhaps fourteen percent compared to six percent for the full disorder.
People with this milder form experience the winter blues without meeting the criteria for major depression. They might feel somewhat sluggish and down during the dark months without becoming seriously impaired. For many of these people, simpler interventions work well: getting outside during daylight hours, exercising regularly, and spending time in well-lit environments.
The distinction between normal seasonal variation in mood, subsyndromal SAD, and full seasonal affective disorder exists on a continuum. Even people with no diagnosable condition tend to feel at least slightly different as the seasons change—more introspective in autumn, more expansive in spring. The connection between human mood and the rhythms of light and darkness seems to be fundamental to our nature, just expressed more strongly in some people than in others.
The ADHD Connection
Recent research has uncovered a surprising link between seasonal affective disorder and attention-deficit/hyperactivity disorder. A study from the National Institutes of Health found that people with ADHD—attention deficit hyperactivity disorder—were about three times more likely to experience SAD symptoms than the general population.
This connection makes a certain biological sense. Both conditions involve disruptions in circadian rhythms and both involve neurotransmitter systems including dopamine and serotonin. People with ADHD often have irregular sleep patterns and difficulty maintaining consistent daily routines—exactly the kind of circadian instability that seems to underlie seasonal depression.
The overlap creates a treatment challenge. Someone who is already struggling with focus and motivation from ADHD may find winter depression devastatingly compounding. The sluggishness and withdrawal of SAD adds to the existing challenges of ADHD, potentially creating a spiral of dysfunction that's hard to escape without addressing both conditions.
Moving Forward
Despite decades of research, some fundamental questions about seasonal affective disorder remain unsettled. A provocative analysis from the Centers for Disease Control and Prevention in 2016 found no links between depression, seasonality, and sunlight exposure in a large population survey—calling the entire concept of SAD into question, at least in its most straightforward form.
Perhaps the condition is real but more complex than simple light deprivation. Perhaps it affects only certain vulnerable individuals rather than following predictable geographic patterns. Perhaps our understanding of the mechanisms remains incomplete.
What seems clear is that human beings are not designed for the artificial environments we've created. We evolved under open skies, our biology tuned to the rhythms of sunrise and sunset, our behavior calibrated to seasons of abundance and scarcity. We now spend most of our time indoors, under artificial lights orders of magnitude dimmer than natural daylight, eating the same foods in every season, expected to maintain the same productivity in January as in July.
For most people, this works well enough. For others, their bodies rebel, insisting through fatigue and craving and despair that winter should be a time of rest. Whether we call this a disorder or simply a mismatch between biology and modern life may be less important than recognizing that help is available—whether that comes from a light box at breakfast, a prescription from a psychiatrist, or simply more time spent outdoors in whatever winter light is available.
The darkness will always return. But we don't have to face it alone, and we don't have to face it unequipped.