Mental Health

Feeling Worse in Winter? The Science Behind Seasonal Affective Disorder (SAD).

Hello my dear readers,

As the nights draw in and the trees continue to lose their leaves, winter is inevitably approaching. While some people enjoy the dropping of temperatures, the magic of Christmas and the chance to finally wear that knitted jumper that’s been sitting at the back of the cupboard, winter can present a challenging time for many.

Seasonal Affective Disorder (S.A.D), sometimes known as winter depression, is a type of depression or bipolar disorder that is experienced at particular times of the year or during particular seasons, most commonly during the winter months. This usually means that during the winter, and sometimes autumn, symptoms of depression either begin or worsen.

The experience of S.A.D is estimated to be between 1% and 10% across the globe and is seemingly dependent on latitude[1]. Unfortunately, I couldn’t find any published studies estimating how many people in the UK experience S.A.D however some research conducted by The Weather Channel and YouGov in 2014 suggested that 29% of the population suffer from some form of S.A.D and that approximately 59% of adults feel generally worse in the winter. According to the Royal College of Psychiatrists women seem to be affected three times more than men, and it is less common in children and older adults.

The symptoms experienced can make this time of year a big struggle for many people. Winter depression presents with many of the symptoms associated with typical depression, such as depressed mood, lack of energy and interest, low concentration, fatigue (especially in the mornings), hypersomnia (excessive daytime sleepiness), reduced sexual interest, and increased hunger[2].

Unfortunately, while there are many hypotheses about why the change in seasons cause depression, there is not sufficient evidence backing up any one of them. The most plausible hypothesis to date suggests that a disturbance to the body’s biological clock by the loss of light during the winter months is to blame.

What is the biological clock and why is it relevant?

Our brains are synchronised with what is going on in our environment. It uses cues known as ‘zeitgebers’ (German for “time givers”) to help regulate certain bodily functions. Our bodies have many cyclical functions with one of them being our sleep-wake cycles. This cycle is a 24-hour routine known as a circadian rhythm and is maintained by our internal clock as well as daylight (an extremely important zeitgeber). It is believed that the decreased exposure to bright daylight during autumn and winter months contributes to the development of winter depression.

Part of the evidence for this hypothesis comes from the efficacy of Light Therapy (LT). Light therapy is suggested treatment for S.A.D, whereby the sufferer is exposed to an artificial bright light for 10 to 15 minutes per day. The current evidence highlighted in a recent systematic review [2] suggests that morning LT but not evening LT is beneficial for enhanced sleep quality and improved daytime activity. Interestingly, morning LT, afternoon LT and evening LT are all as equally beneficial for improvement of the actual mood symptoms.

Although the evidence demonstrates a clear involvement of our circadian rhythm, the underlying mechanism, how reduced daylight actually equals depression, is still unknown.

Other Current Insights to S.A.D

The Weather

A study from 2017 investigated the effect of weather conditions on people diagnosed with S.A.D. during the winter months[3]. Data for 36 different types of weather observation over a 6-year period (2003-2009) was used. Measurements included sunshine duration, global radiation, horizontal visibility, cloud clover and mist. The researchers found that while no one weather influenced depression symptoms, a ‘mix of meteorological ingredients’ did. They found distinct links between S.A.D symptoms and luminance-related weather (aka weather that affected light levels) and that weekly weather was as important to S.A.D symptoms as the seasonal change is.


A 2016 study[4] explored the impact of vegetarianism on S.A.D, finding that the percentage of people suffering from S.A.D was 4 times higher in Finnish vegetarians than in non-vegetarians. In a Dutch outpatient clinic, the percentage of vegetarians among the S.A.D patients was 3 times higher. While there is no established understanding for why the higher prevalence of S.A.D among vegetarians exists, the authors speculate that the relationship may be due to the amount of protein consumed. Protein in meals can influence the metabolism of serotonin (‘the happy hormone’) and concentration of tryptophan (an essential amino acid which can be converted into serotonin) in the brain. It could be that there is generally less protein in vegetarian diets, but how and if this relates to the prevalence of S.A.D in vegetarians is not yet known.

Another popular hypothesis is that Vitamin D is involved in the development of S.A.D. An important source of Vitamin D is the sun via ultraviolet light. In the summer, there is a high synthesis of Vitamin D via the sunlight on the skin however from October through to March, it is much lower. There has been a relatively small amount of studies regarding Vitamin D supplementation on well-being, however most have found no effect of Vitamin D on depression. A randomised placebo-controlled double-blind trial in 2014 also failed to demonstrate any impact of Vitamin D on S.A.D symptoms[5]. This is not to say however that there is no effect to be found, the authors of the study point out there are flaws with the study and further exploration into the matter is needed.


Concerning genetics, I came across one recent study exploring a genetic component to S.A.D.[6]. The authors conducted a meta-analysis of genome-wide association studies (GWAS).

For those who are reading with little knowledge about what a GWAS is I will expand. GWAS studies are extremely important in research, especially when there is little prior knowledge about the genes involved with certain disorders. Instead of deciding on a specific gene and its variants to explore (candidate gene), the whole genome is investigated. Essentially, you have your population, and you take each person’s genome and compare it to everyone else’s to see if there are common denominators for a specific disorder or disease. If any variants stand out, then these could be markers that could be used to predict the disease.

The study found that there was one specific potential candidate gene for susceptibility to S.A.D and that further study was needed to confirm the association.

What Does All of This Mean Then?

Unfortunately, we are still pretty far behind in truly understanding how S.A.D arises and why. As I’ve demonstrated above, there are many lines of enquiry, with the most popular being the effect of lessening light levels. Fortunately, there are treatments available and there are things that we can do to help manage the way winter affects us.

In a bid not to make this post too long, there will be a second post this week detailing the available treatments, evidence-based ways of keeping well during winter, and personal accounts from individuals on how they look after themselves in winter.

Thank you for reading, and I hope you are all keeping well this winter and lockdown. If you are finding things difficult I have listed some helpful resources below.

MIND – Seasonal Affective Disorder

CALM – Campaign Against Living Miserably

Samaritans -116 123

SHOUT – Mental Health Text Line – 58258

[1] Ybe Meesters and Marijke Cm Gordijn, ‘Seasonal Affective Disorder, Winter Type: Current Insights and Treatment Options’, Psychology Research and Behavior Management, 9 (2016), 317–27 <;.

[2] Giulia Menculini and others, ‘Depressive Mood and Circadian Rhythms Disturbances as Outcomes of Seasonal Affective Disorder Treatment: A Systematic Review’, Journal of Affective Disorders, 241 (2018), 608–26 <;.

[3] Christophe Sarran and others, ‘Meteorological Analysis of Symptom Data for People with Seasonal Affective Disorder’, Psychiatry Research, 257 (2017), 501–5 <;.

[4] Alie N. R. Meesters and others, ‘Is There a Relationship between Vegetarianism and Seasonal Affective Disorder? A Pilot Study’, Neuropsychobiology, 74.4 (2016), 202–6 <;.

[5] Tenna Bloch Frandsen and others, ‘Vitamin D Supplementation for Treatment of Seasonal Affective Symptoms in Healthcare Professionals: A Double-Blind Randomised Placebo-Controlled Trial’, BMC Research Notes, 7 (2014), 528 <;.

[6] Kwo Wei David Ho and others, ‘Genome-Wide Association Study of Seasonal Affective Disorder’, Translational Psychiatry, 8.1 (2018), 190 <;.


2 thoughts on “Feeling Worse in Winter? The Science Behind Seasonal Affective Disorder (SAD).”

  1. It’s a form of bipolar disorder? I hadn’t heard that before. As I mentioned in the tweet, I love fall, getting out the warm sweaters and LOVE the Xmas season!!! But I do mourn the loss of sunlight. Still, my worst symptoms happen during spring and early summer. Weird, right? I do have allergies, but would that be why?

    Liked by 1 person

    1. Hi there! Yes, for people with bipolar disorder, the winter months can be a trigger for a season of depression. That is interesting, I actually did read something about histamine but I didn’t read enough to be able to comment on it 🙂 I love autumn too and warm sweaters!


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