Don’t overlook the possibility that your patient may be experiencing Seasonal Affective Disorder (SAD) — a common and often under-recognised winter condition. SAD is characterised by a recurrent, seasonal pattern of major depressive episodes, typically beginning in autumn or winter and easing as the days lengthen in spring and summer. While it affects an estimated 3–8% of the population, milder versions — the so-called “winter blues” — are even more widespread.

Because many people assume that everyone feels a bit miserable in the dark months, SAD is frequently dismissed or goes undiagnosed. But there’s a clear and important difference between normal winter fatigue and a true seasonal depressive episode, and it’s worth clinicians actively keeping this on their radar. 

I have compiled some information to help you understand the condition in patients/relatives, or yourself. 

Why Does Sad Happen?

Reduced daylight → circadian rhythm disruption 

Shorter days and longer nights impact the brain’s suprachiasmatic nucleus, destabilising sleep–wake cycles, energy regulation, and mood. 

Melatonin overproduction

With prolonged darkness, the pineal gland produces more melatonin, leading to: 

  • hypersomnia 
  • low energy 
  • difficulty getting up in the morning 
  • “Hibernation-like” behaviour 

Serotonin dysregulation

Lower sunlight levels reduce serotonin synthesis and alter serotonin transporter activity. This contributes to: 

  • low mood 
  • carbohydrate cravings 
  • emotional heaviness 

Vitamin D insufficiency 

While evidence is mixed, low vitamin D may play a role for some patients, especially in northern climates. 

Typical Clinical Presentation

Patients often describe winter depression as feeling “slowed down” or “foggy.” Core features include: 

  • marked fatigue / low energy 
  • increased sleep (but not necessarily restorative) 
  • carbohydrate and sugar cravings 
  • weight gain 
  • low mood, tearfulness, irritability 
  • loss of interest and motivation 
  • difficulty concentration 
  • social withdrawal (“I just want to stay home and shut the world out”) 

The atypical depression profile (hypersomnia, increased appetite) is classic for SAD. 

Screening in Primary & Urgent Care

Quick useful questions: 

  • “Do your symptoms follow a seasonal pattern?” 
  • “Do you feel significantly better in spring/summer?” 
  • “Has this happened at the same time in previous years?” 
  • “Does it feel like your body wants to hibernate?” 

Tools: 

  • PHQ-9 still appropriate. 
  • SIGH-SAD (Structured Interview Guide for SAD) if needed in specialist settings. 

Differential Diagnosis to Keep in Mind

  • Hypothyroidism 
  • Iron deficiency 
  • Anaemia 
  • Chronic fatigue 
  • Bipolar disorder (watch for seasonal mania/hypomania) 
  • Alcohol/drug use 
  • Grief anniversaries linked to a specific time of year 

Evidence Based Management

Light Therapy (First-line for many) 

    • Use a 10,000-lux light box, 20–30 minutes each morning. 
    • Can improve symptoms within 1–2 weeks. 
    • Clinical tip: Patients should use it early in the day — using it late can disrupt sleep. 

    CBT-SAD

      A specialised cognitive behavioural therapy designed for winter depression. Evidence suggests CBT-SAD is as effective as light therapy short-term, and more effective for long-term prevention. 

      Antidepressants 

        SSRIs are commonly used. Most evidence supports: 

        • Sertraline 
        • Fluoxetine 
        • Bupropion XL (in some regions, it’s licensed specifically for SAD prevention) 

        Consider if: 

        • Symptoms are moderate–severe 
        • significant functional impairment 
        • poor response to light alone 

        Behavioural & Lifestyle Interventions 

          These matter more for SAD than for some other depression types: 

          • daily outdoor exposure (even 10–15 minutes) 
          • structured daytime routine 
          • regular exercise (morning is best) 
          • sleep regulation 
          • reducing alcohol (a common December/January exacerbator) 

          When to Worry 

          Red flags: 

          • acute suicidal ideation 
          • severe functional decline 
          • agitation or signs of mixed affective states 
          • Symptoms that don’t fit the seasonal pattern 
          • new-onset depression in older adults (consider medical causes) 

          The Holiday Twist 

          In December specifically, SAD often blends with: 

          • holiday stress 
          • financial pressure 
          • disrupted routines 
          • late-night socialising 
          • alcohol use 
          • grief anniversaries 

          This means patients may underestimate their symptoms or misattribute them to “holiday overwhelm.” 

          A simple line that often resonates: 
          “It might not just be holiday stress — this could be your seasonal pattern showing up.” 

          For more information visit – Treatment – Seasonal affective disorder (SAD) – NHS