The Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) defines seasonal affective disorder (SAD), also called seasonal depression, as a psychiatric diagnosis with symptoms that peak during a specific season and then recede toward the end of the season. Similarly, the Mayo Clinic defines SAD as a form of depression related to seasonal change.
Research shows about 6 percent of the U.S. population experiences major SAD, while around 14 percent of the population experiences minor SAD. Although most people associate seasonal affective disorder with depressive symptoms that appear during winter, evidence shows that SAD can appear in any season, and its onset is related to various individual factors.
This article – although it is about the effect of the seasons on psychiatric disorders and mental health diagnoses – is not about SAD, per se. Rather, it’s about the effect the seasons have on the symptoms of various mental health diagnoses and how the seasons impact rates of suicide among people with mental health diagnoses and/or alcohol/substance use disorders.
The peer-reviewed journal study, “Seasonality Of Brain Function: Role in Psychiatric Disorders,” published in February 2023, examined results from all the available research on the relationship between the seasons and the symptoms of mental health and psychiatric diagnoses, including seasonal depression.
Here’s how they describe their research effort:
“Understanding the mechanisms that account for inter-individual variations in seasonality is relevant to the development of individualized prevention and treatment for psychiatric disorders. Despite promising findings, seasonal effects are still understudied and only controlled as a covariate in most brain research.”
What they mean by that scientific language is this: if we understand how different seasons affect the symptoms of mental health disorders in different people, then we can target their treatment to address these variations, and better support them reach their treatment and recovery goals.
Our Natural Clock, Seasonal Affective Disorder, and Mental Health
The study authors begin by explaining the evidentiary basis for their research. Over the years, earlier researchers published data on the effect of circadian rhythms, our internal circadian clock, and geography – with regards to relative amount of sunlight hours – on rates of psychiatric and mental health diagnoses.
Circadian rhythm or circadian clock is defined as “physical, mental, and behavioral changes that follow a 24-hour cycle and respond primarily to light and dark.” The easiest circadian rhythm to identify in humans is our sleep patterns: we tend to sleep at night and be active during the day.
- Geographic models suggest genetic adaptions of the human circadian clock may relate to the prevalence mood disorders
- Prevalence of psychiatric diagnoses increases with distance from the equator, including:
- Major depression
- Schizophrenia
- Suicide attempts in bipolar disorder increases with distance from the equator
- Severity of depressive symptoms increases with distance from the equator
- Malfunction of the biological adaptions to environmental challenges e.g., prominent light changes in high-latitude regions might increase the vulnerability to certain psychiatric disorder
The researchers also recognize that seasonal social factors and seasonal stress may also play a role in symptoms, but the evidence suggest that biological factors have a greater, critical role in the seasonality of mental health symptoms.
Psychiatric Diagnoses and Mental Health: The Impact of the Seasons
After a thorough review of the available data, the researcher identified the following trends in the seasonality of mental health symptoms and psychiatric diagnoses.
Seasonality of Mental Health/Psychiatric Diagnoses
- Major depressive disorder (MDD):
- Depressive symptoms peak in winter
- Manic (subthreshold) episodes peak in spring and summer, with a small peak in autumn
- Mixed episodes peak in early spring and late summer
- Bipolar disorder I & II:
- Depressive symptoms peak in winter
- Manic and hypomanic episodes peak in spring and summer, with a small peak in autumn
- Mixed episodes peak in early spring and late summer
- Schizophrenia:
- Initial onset of symptoms most often appears in the middle of winter
Study authors note that extreme variations in seasonality of symptoms is more common among patients with severe depression or bipolar 1 than patients with moderate depression or bipolar 2. Among patients with schizophrenia, researchers have not identified predictable yearly peaks and valleys – i.e. seasonality – in symptoms like hallucinations, delusions, social withdrawal, loss of motivation, and flat affect.
In addition to seasonal changes in symptoms and symptom severity among people with mental health diagnoses, researchers also identify patterns in suicidal behavior and drug overdose.
Seasonality of Suicidality Among People With Mental Health Disorders and Substance Use Disorder
- Major depressive disorder (MDD):
- Suicide attempts peak in spring and summer, with a small peak in autumn
- Suicide rates increase with distance from the equator
- Bipolar disorder I:
- Suicide attempts peak in summer
- Alcohol use disorder:
- Suicide attempts peak in spring
- Opioid use disorder:
- Suicide attempts peak in summer
- Suicide attempts increase with proximity to equator
- Intentional fatal overdose, all drugs and substances:
- Rates of intentional fatal drug overdose peak in spring and summer
The data make it clear: symptoms of some mental health diagnoses vary with the seasons and rates of suicide vary with the seasons. In general, data shows that among people with depressive disorders, severity of symptoms increase with distance from the equator, and suicidality increases in warmer months. Rates of suicide among people with bipolar 1, alcohol use disorder, and opioid use disorder increase during the summer, as do overall rates of intentional drug overdose.
What causes these seasonal changes?
Seasonal Variation in Mental Health and Psychiatric Diagnoses: Possible Causes
The study authors identified several potential causes for the seasonal variability in the symptoms of mental health diagnoses, and created a matrix of sorts for understanding these changes. We’ll share the potential causes, then discuss the matrix – or flow chart, for lack of a better phrase – they created to describe the impact of these causes on an individual with a mental health disorder.
The potential causes they identified are:
- Seasonal changes in neurotransmitters
- Seasonal changes in brain function and structure
- Immune system variation
- Circadian rhythm variation
1. Seasonal Changes in Neurotransmitters
Neurotransmitters are the chemical messengers in our brain that play a role in the way we think, act, and feel. Researchers identified changes in:
- Dopamine:
- Postmortem studies on people who died in winter show lower levels of various dopamine transport molecules compared to people who died in summer
- Positron emission tomography (PET) scans show lower dopamine levels in people with winter SAD
- PET scans on people without mental health disorder show higher concentrations in dopamine in summer, compared to winter
- Serotonin:
- Postmortem studies on people who died in winter show lower levels of serotonin in key brain areas, compared to people who died in summer
- Studies on healthy (living) subjects showed lower levels of serotonin in key brain areas in winter as compared to summer
- Studies show an increase in serotonin levels with an increase in sunlight hours, i.e., as the days get longer, and winter moves toward summer
2. Seasonal Changes in Brain Function and Structure
The effect of the seasons on brain function and structure has not been studied as extensively as seasonal changes in neurotransmitters. However, the available research shows the following:
- Studies show metrics like attention, executive function, and decision-making are more robust when tested in spring and summer, as compared to fall and winter
- Studies on health adults reveal positive associations between the amount of daylight hours and the measured volume in brain areas related to emotion and emotional processing
- Preliminary studies show the volume of various brain areas may vary with the seasons, but more research is needed in this area
3. Immune System Variation
Studies show that genes related to the immune system display seasonal variation, and that changes in immune function may be connected to changes in brain structure in people with psychiatric disorders. While no seasonal variation in these changes have been identified in research, here’s the relevant information that makes these observations relevant:
- Immune dysfunction has a negative impact on cognition and emotional control
- Immune-related inflammation is associated with poor cognitive performance
4, Circadian Rhythm Variation
Circadian rhythms, by definition, mean variation on a 24-hour cycle, mostly related to light and dark, and mostly related to the presence of various hormones and neurotransmitters produced and release in response to relative levels of light and dark. These variations occur daily, and the broad parameters of these daily changes are associated with the changes in the seasons. Here’s how those changes might impact mental health diagnoses and psychiatric symptoms:
- Neurotransmitter levels – i.e. the dopamine and serotonin we mention above – impact circadian rhythm in humans
- Serotonin can both increase and decrease light-induced changes in the brain
- Dopamine decreases light-induced changes in the brain
- Disruptions in neurotransmitter systems can have a negative impact on light-induced changes in the brain
- Irregular circadian rhythms affect the sleep and smooth overall function, which can increase the likelihood of developing depression, anxiety, bipolar disorder, and seasonal affective disorder
Putting it All Together: Seasonal Impact on Mental Health Disorders
In addition to the impact of neurotransmitters, seasonal changes in the brain, variation in the immune system, and circadian rhythm disruption, the study combined their existing knowledge with their review of available research and created a matrix/flow chart that shows four primary factors – in addition to those we discussed above – affect psychiatric symptoms and the onset of mental health diagnoses.
These four factors include:
- Biological factors:
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- Age: younger people report greater seasonal variation in symptoms compared to older people
- Gender: women show greater seasonal variation in symptoms compared to men
- Timing and amount of exposure to light, which are affected by:
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- Social factors, like work schedules, social schedules, and holidays
- Personal factors, such as the amount of time spend indoors
- Social interactions:
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- Stress: increased stress is associated with increased rates of psychiatric disorders and symptom severity
- Available support: social support decreases rates of psychiatric diagnoses and symptom severity
- Exposure to drugs: exposure to drugs of misuse increases rates of psychiatric diagnoses and symptom severity
- Environmental factors
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- Temperature: in general, warm temperatures are associated with lower rates of depression, as shown by the evidence that rates of mental health diagnoses increase with distance from the equator
- Available sunlight: higher levels of daily sunlight are associated with lower levels of mental health diagnoses and decreased symptom severity
This valuable information clarifies the impact of the seasons on mental health diagnoses, and confirms what preliminary data suggest: mental health diagnoses and symptom severity are related to the seasons, connected primarily by the levels of available sunlight and the impact that has on various social and biological factors.
How This Information Helps
The data generated by this study can help us offer people with mental health diagnoses more comprehensive, sophisticated, and effective treatment. If we understand how the factors above – biological, social, environmental, levels of sunlight, and timing of levels of sunlight – manifest for each individual we treat, then we can better understand their diagnosis and its expected trajectory.
For instance, we can help patients with depressive disorders plan for periods of the year when their symptoms escalate: that will mean they’re ready to deploy the coping mechanisms they develop during treatment ahead of time. In addition, we can use the information from this study to learn all we can about the origin and development of any mental health diagnosis in our patients: the more we know about all the factors at play in the lives of our patients, the better we can help them meet their treatment goals.