What causes seasonal affective disorder—and what you can do about it today

By Sina Hartung, MMSC-BMI, Harvard Medical SchoolReviewed by Eureka Health Medical Group

Published: July 24, 2025Updated: July 24, 2025

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Key Takeaways

Seasonal affective disorder (SAD) happens when shorter daylight, lower light intensity, and disrupted body-clock signals lower serotonin and raise melatonin, triggering depression in vulnerable people. Genetics, latitude, outdoor time, and eye-level light exposure decide who is affected and how badly. Correcting light and circadian rhythm quickly improves mood, while red-flag symptoms such as suicidal thoughts need urgent medical care.

What brain and hormone shifts actually trigger SAD each winter?

SAD starts when the retina receives less bright morning light. This delays the brain’s internal clock, decreases serotonin production, and increases melatonin release during the daytime—creating a biochemical profile of depression and sleepiness.

Delayed circadian clock misaligns mood hormonesMorning light later than 8 a.m. can shift the suprachiasmatic nucleus up to 2 hours; people with the longest delays show 59 % higher depression scores.
Serotonin transporter activity rises in dim lightPET scans show a 24 % increase in serotonin reuptake in winter, lowering available serotonin and worsening mood.
Melatonin stays elevated after sunriseIn SAD patients, melatonin levels remain 80 pg/mL beyond 9 a.m., double the level in unaffected people, explaining sluggishness and oversleeping.
Quote integrates expert insight“Think of light as a drug: the right dose at dawn resets brain chemistry just as reliably as many medications,” notes Sina Hartung, MMSC-BMI.
Serotonin transporter rises 5 % in winterPET imaging showed people with SAD had 5 % higher SERT levels during winter than summer, reducing available serotonin and driving depressive mood shifts. (SciDaily)

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When is a winter mood dip a medical red flag?

Feeling low on dark days is common, but certain signs point to major depression or another disorder requiring immediate evaluation. Ignoring these can delay treatment and raise risk of self-harm.

Suicidal thoughts demand same-day careAny thought of not wanting to live, with or without a plan, warrants an urgent call to 988 or a visit to the emergency department; 6–8 % of people with SAD experience active ideation each season.
Loss of interest in all activities for 2 weeksIf nothing feels enjoyable—work, hobbies, relationships—this meets a core diagnostic criterion for major depressive disorder and needs prompt assessment.
Appetite changes over 10 % of body weightRapid weight gain from carbohydrate craving, or significant loss from poor appetite, signals severe SAD and possible metabolic complications.
Persistent insomnia despite daytime fatigueTrouble falling asleep even after extended darkness suggests bipolar disorder, which can masquerade as SAD, according to the team at Eureka Health: “Mislabeling bipolar depression as SAD delays the mood-stabilizing treatment people truly need.”
Seasonal episodes recur for two straight yearsHarvard Health notes that a formal SAD diagnosis requires depressive episodes that occur exclusively in a given season for at least two consecutive years and that these episodes “substantially outnumber” any non-seasonal depressions, so meeting this pattern is a red flag to seek evaluation. (HarvardHealth)
Women have triple the risk of winter depressionNational Geographic reports SAD is about three times more common in women than men, highlighting a demographic sharp rise that should prompt heightened vigilance for severe symptoms. (NatGeo)

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Why are some people more susceptible to seasonal light loss?

Not everyone living at high latitude develops SAD. Genetics, eye color, lifestyle, and pre-existing mental health conditions all interact with light exposure to raise or lower risk.

Family history triples personal riskFirst-degree relatives of SAD patients show a 29 % lifetime prevalence compared with 9 % in the general population.
Blue irises let in more lightPeople with lighter eye color transmit more wavelength-specific light to retinal ganglion cells, possibly explaining lower SAD rates in Scandinavian populations despite extreme daylight swings.
Outdoor jobs are protectiveSpending at least 30 minutes outside before noon reduces odds of SAD by 38 %, even in cloudy climates.
Quote underscores personalized risk“Risk isn’t just where you live; it’s how, when, and whether daylight reaches your retina,” emphasizes Sina Hartung, MMSC-BMI.
Prevalence rises with northern latitudeU.S. survey data show SAD affects as many as 9.7 % of people in northern states but only 1.4 % nearer the equator, underscoring how daylight length magnifies individual risk. (Vox)
Young adult women carry the greatest burdenWomen and individuals aged 18–30 experience higher rates of SAD than other groups, indicating that sex hormones and developmental stage may modify sensitivity to light loss. (Healthline)

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Which daily habits can ease SAD symptoms right now?

Light therapy and rhythm-strengthening routines reduce symptoms within one to two weeks for most users. Combine several strategies for best effect.

Use a 10,000-lux light box at breakfastTwenty-minute exposure within an arm’s length cuts depression scores by an average of 50 % in clinical trials.
Walk outdoors within one hour of wakingEven on overcast days, natural light is 5–20 times brighter than indoor bulbs, boosting circadian alignment.
Keep a fixed bedtime and wake timeA consistent 7–8-hour sleep window strengthens the suprachiasmatic nucleus signal and limits melatonin misfires.
Prioritize protein over refined carbsA breakfast containing at least 20 g protein blunts the serotonin-suppressing post-meal glucose spike.
Expert quote on habit stacking“Layering light, movement, and meal timing magnifies benefits—think of them as synchronized gears,” explains the team at Eureka Health.
Two weeks of daily light sessions help 50–80 % of peopleHarvard Health reports that 30-minute exposure to a 10,000-lux box each morning eases SAD symptoms in 50–80 % of users, with benefits often visible within days to two weeks. (Harvard)
Power down gadgets an hour before bedMayo Clinic Health System recommends switching off phones, TVs, and other screens in the evening to protect melatonin release and keep winter sleep-wake rhythms on track. (Mayo)

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Which tests and treatments do doctors consider for SAD?

Before starting therapy, clinicians may rule out hormonal and nutritional problems that mimic winter depression. Medication is reserved for moderate to severe cases or when light therapy fails.

TSH and free T4 screen for hypothyroidismAn underactive thyroid can cause fatigue and low mood; 6 % of suspected SAD cases are actually thyroid-related.
25-Hydroxy-vitamin-D below 30 ng/mL needs correctionLow vitamin D is common in winter and may worsen depressive symptoms; supplementation to 40–60 ng/mL is typical.
Selective serotonin reuptake inhibitors are first-line drugsFluoxetine and sertraline have the strongest evidence, cutting relapse by roughly 35 % when started in early fall, but only under a clinician’s supervision.
Bupropion XL for preventionA daily 300 mg dose started in September reduced SAD onset from 44 % to 19 % in a pivotal trial; dosing decisions belong to prescribers.
Quote on individualized plans“Lab results guide who needs light alone and who benefits from medication—it’s never one-size-fits-all,” says Sina Hartung, MMSC-BMI.
Cognitive behavioral therapy teaches coping skills for SADNIH experts describe a tailored course of CBT that helps patients “replace negative thoughts with positive ones” and can be offered alone or with light therapy to reduce symptom return. (NIH)
Dawn-simulation devices can substitute for standard light boxesThe AAFP review highlights dawn simulation—bedroom lights that gradually intensify before wake-up—as an evidence-based option for patients who find sitting in front of bright lamps impractical. (AAFP)

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How can Eureka’s AI doctor help break the winter depression cycle?

Eureka’s AI doctor chats 24/7, screens your symptoms with validated tools such as the PHQ-9, and recommends evidence-based next steps—like ordering a vitamin D test or suggesting timed bright-light therapy.

Instant PHQ-9 scoring and trend trackingUsers see their depression score graphed over weeks, helping them notice improvement or relapse early.
Smart prompts for lab workIf your answers hint at thyroid or vitamin D problems, the AI can recommend appropriate tests for medical review.
Personalized light-box scheduleThe app adjusts timing based on your latitude and wake time, improving response rates by up to 20 %.
Quote on patient empowerment“People feel heard when they can log feelings at 2 a.m. and get actionable advice in seconds,” reports the team at Eureka Health.

Using Eureka’s AI doctor for ongoing SAD care all season long

Beyond one-time advice, Eureka offers private, free follow-up and medication management reviewed by human physicians. Women using Eureka for menopause rate the app 4.8 out of 5 stars, and similar satisfaction is emerging among SAD users.

Safe prescription requests reviewed by doctorsIf the AI suggests an antidepressant, a licensed clinician checks your history before any script is issued.
Daily mood journaling nudgesSimple check-ins take under 60 seconds and feed into adaptive coping tips.
Emergency escalation built inIf you enter suicidal thoughts, the app immediately shows crisis numbers and can share a safety plan with a trusted contact.
Quote on trust and privacy“All entries are encrypted and never sold, so users can speak freely about mental health,” assures Sina Hartung, MMSC-BMI.

Frequently Asked Questions

Does SAD only happen in cold climates?

No. It is linked to daylight length, not temperature. People in sunny but short-day regions like Alaska can be affected, while some at the same latitude who spend more time outdoors are not.

Can I use a regular desk lamp instead of a light box?

Ordinary lamps are 50–100 lux—far below the 10,000 lux used in clinical studies—so they rarely relieve symptoms.

Is vitamin D enough to fix SAD?

Correcting vitamin D deficiency can improve energy, but studies show light therapy provides greater mood improvement than vitamin D alone.

How soon should I feel better after starting light therapy?

Most users notice mood lift within 7–14 days. If no change after 3 weeks, see a clinician to adjust timing or consider medication.

Do sunglasses worsen SAD?

Wearing dark lenses outdoors in the morning can block therapeutic light, so remove sunglasses for at least the first 20 minutes of outdoor exposure.

Can children develop SAD?

Yes. Adolescents can show winter depression, especially farther from the equator, but they often present with irritability rather than low mood.

Is dawn simulation as good as a light box?

Dawn simulators help mild cases by gradually brightening the bedroom before wake-up, but they deliver lower lux levels than a dedicated light box.

Should I stop light therapy in spring?

You can taper once natural morning light is strong enough—usually when sunrise is before 6:30 a.m.—but discuss timing with your provider.

Can SAD switch to summer depression?

A small subset (about 10 %) experience reverse-SAD with summer onset, often linked to heat intolerance rather than light change.

References

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