SAD Causes

Medically reviewed by:

Dr Dan Bunstone - Medical Officer

Updated: Aug 18, 2021

Next review: Aug 18, 2022

SAD is a type of depression, but the exact causes of it remain largely unknown. It typically begins and ends at the same time each year and so there are certain factors that are believed to play a part.

While the causes of SAD of not fully understood, it is known that a higher percentage of females suffer from it compared to men. There could also be links to family history and a greater number of cases in people who have another form of depression or a bipolar disorder.

The reduced levels of sunlight during autumn and winter may disrupt your biological clock, known as your circadian rhythm.

Less sunlight during the daytime means that less light hits the back of the eye. When light hits the back of the eye, messages are sent to the parts of the brain that controls appetite, sleep, mood, activity and sex drive, so if you are not getting enough light, it could cause adverse effects, such as SAD.

As the condition only affects some people and not others, it seems that some people may need more light than others, in order to keep their biological clock in order.

The brain chemical (neurotransmitter) serotonin is known to affect your mood, sleep and appetite, so a drop in serotonin levels in your body due to a lack of sunlight may be one of the causes of SAD.

When it is dark, the body creates melatonin, and this helps us sleep. People with SAD may produce more melatonin than others, contributing to SAD.

The onset of autumn and winter usually triggers SAD, but it may also be triggered by the opposite seasons. In these rarer cases, Reverse Seasonal Affective Disorder, or summer depression, results in depression during spring and summer.

MIND, Understanding Seasonal Affective Disorder (SAD), 2016.
mind.org.uk/media/4694765/understanding-sad-2016-pdf.pdf
E, Roth & Legg, T. Healthline, Seasonal Affective Disorder (Major Depressive Disorder with Seasonal Pattern), 08/11/2017.
healthline.com/health/seasonal-affective-disorder
NHS Inform Scotland, Seasonal affective disorder (SAD): Causes, 20/02/2018.
nhsinform.scot/illnesses-and-conditions/mental-health/seasonal-affective-disorder-sad#causes
Mayo clinic, Seasonal affective disorder (SAD): Symptoms & Causes, 25/10/2017
mayoclinic.org/diseases-conditions/seasonal-affective-disorder/symptoms-causes/syc-20364651
Nordqvist, C. Medical News Today, Seasonal affective disorder (SAD): What is it?, 24/07/2017.
medicalnewstoday.com/articles/10306.php

Dr Dan Bunstone - Medical Officer

What causes Seasonal Affective Disorder (SAD)?

Circadian rhythm

Low serotonin levels

High melatonin levels

Reverse Seasonal Affective Disorder

Mental Health Articles