Sunlight helps to regulate the hormones serotonin and melatonin, it has also been shown to affect brain blood flow. Thus, sunlight may not only affect mood directly, but also via its effect on cognition, because cognitive impairment can underlie a depressive episode.
A lack of vitamin D, which is synthetized in the presence of sunlight, can lead to an increase in low mood, fatigue, and depressive episodes. On the other hand, it is also important to keep in mind that too much sunlight can also have detrimental health effects.
Time Magazine has followed the trail of the benefits of sunlight in “Why Sunlight Is So Good For You“. HealthLine provides an overview in “What Are the Benefits of Sunlight?” There is a long list of conditions, psychological and bodily, that have been associated with a lack of sunlight.
A type of depression that is directly related to the seasons is Seasonal Affective Disorder (SAD). People who have this type usually are more inclined towards low mood in the winter months when there is less sunshine. You may want to read further on this topic in the article Seasonal affective disorder (SAD) published by the Mayo Clinic.
There is also some empirical evidence that a lack of sunlight can decrease our cognitive functioning, our ability to think about problems and concentrate on them. One study titled “Effect of sunlight exposure on cognitive function among depressed and non-depressed participants” sought to demonstrate this.
Seasonal Affective Disorder in Ireland
Seasonal affective disorder, often shortened to SAD, refers to a recurring seasonal pattern of depression. For many people this means mood, energy and motivation worsening in autumn or winter and improving again in spring or summer.
In Ireland, shorter winter days, long stretches indoors, weather, reduced outdoor activity and social isolation can make seasonal mood changes more noticeable. Not every winter dip is SAD, but recurring seasonal depression deserves attention.
Common Signs
SAD can include low mood, loss of interest, tiredness, oversleeping or poor sleep, changes in appetite, carbohydrate cravings, difficulty concentrating, irritability, social withdrawal, hopelessness and reduced motivation.
The important pattern is recurrence. If symptoms return around the same season and affect functioning, it is worth discussing this with a GP or mental-health professional.
What May Help
Treatment discussions may include light therapy, psychotherapy such as CBT adapted for seasonal depression, antidepressant medication, routine, outdoor daylight, movement, social contact and attention to sleep. Vitamin D is often discussed, but evidence as a SAD treatment is mixed, so it should not be presented as a simple cure.
Light therapy may be helpful for some people, but it should be discussed with a clinician if there is bipolar disorder, eye disease, photosensitising medication, pregnancy, severe depression or uncertainty about safety.
Therapy Route
Psychotherapy can help with the behavioural and relational effects of seasonal depression: withdrawal, loss of routine, self-criticism, reduced communication, disrupted sleep, hopelessness and the meaning a person gives to winter.
This page should link to depression therapy, depression hub, anxiety therapy, sleep therapy/insomnia and the Dublin/online service page.
Urgent Boundaries
If seasonal depression includes suicidal thoughts, inability to stay safe, psychosis, mania, severe self-neglect or risk to another person, urgent support is needed. In Ireland, call 112 or 999 in immediate danger, or use local emergency/crisis services.
Medication decisions need to be discussed with a qualified prescriber.
FAQ
- Is this page a diagnosis? No. It is educational and cannot diagnose or assess individual risk.
- When should someone seek professional help? When symptoms are persistent, severe, risky, impairing, confusing, or affecting sleep, work, study, relationships or day-to-day functioning.
- What if there is immediate danger? Use local emergency services or crisis support. In Ireland, call 112 or 999 if there is immediate danger.
Education and safety note. This page is for general information. It cannot diagnose you, assess your individual risk, or replace care from a qualified professional. If you are in immediate danger, may harm yourself or someone else, cannot stay safe, or have symptoms that may be medically urgent, contact local emergency services or crisis support. In Ireland, call 112 or 999 or go to the nearest emergency department; you can also read the HSE crisis guidance. Medication decisions need to be discussed with a qualified prescriber.
Sources and review. Published or updated in May 2026. This page is educational and uses public-health, guideline, peer-reviewed, or professional sources where clinical claims are made.