In 2005, psychologist Cliff Arnall came up with a formula which looked at various factors, including winter weather conditions, post-Christmas money woes, the amount of time since Christmas and the fact that we all hate Monday, and decided that the third Monday of January, is the most depressing day of the year.

Since then it has become known as Blue Monday and crops up every year even though it has actually been proved to be a myth. A travel company actually paid for the research Arnall did in an attempt to convince people to book more holidays to cure their winter misery.

However, for some people, the winter blues aren’t a trivial thing.

For most of them, they start around October, with symptoms lifting by April. It manifests as winter doldrums, the “I-can’t-wait-for-winter-to-end” feeling that produces mild but manageable sluggishness and food cravings.

However, 7 per cent of people, according to the HSE, are affected by seasonal affective disorder, which has the rather apt acronym SAD. It is related to changes in the seasons, which interrupt your biological clock’s normal rhythm. It is typically diagnosed after at least two consecutive years of intense symptoms.

SAD is most likely triggered by the lack of sunlight and daylight hours in winter, which affects the hormones – namely melatonin and serotonin – in the part of your brain that controls your mood, sleep and appetite. It is particularly common in people who live furthest from the equator.

The American Psychiatric Association found 10-20 per cent of people in the US feel more depressed during the winter. However, a true diagnosis of SAD is less common, at around 1-10 per cent.

Depressive symptoms

More than simple winter blues, SAD can be a debilitating illness. Symptoms include depression, lack of energy, concentration problems, anxiety, over-eating, loss of libido and sudden mood changes.

SAD is associated with feeling almost a need to hibernate, marking it out as different from “traditional” depression which can have similar symptoms.

With symptoms often beginning at the end of autumn and continuing until early spring, people who suffer with SAD see the same depressive symptoms recurring every year around the same time.

So what can you do if you have SAD?

1. Chase the light

Greater exposure to natural light will help with hormone production if, like most of us, you do not have the option of simply disappearing to hotter climes for the winter months.

Working in an office for hours each day does you no good, so a walk to work in the morning or taking your lunch outside can all help to improve symptoms.

Open your blinds at home and sit by windows when you can; even small changes to get more natural light will help.

2. Light therapy

For people with more severe cases, natural sunlight may not be enough. Light therapy is one of the most popular and effective treatments for SAD and usually begins working within a fortnight.

Sitting in front of a light box for 30-60 minutes makes a big difference. Also, because you just have to face the light rather than look into it, you can be doing other things like reading a book during the therapy.

3. Improve your lifestyle

As with other forms of depression, it can heighten your mood if your lifestyle improves. People with SAD often crave sweets and starchy food in the winter, which although they boost energy briefly, often end up leaving you feeling lethargic.

A healthy diet high in proteins, vegetables and complex carbs, a normal amount of sleep and doing some exercise (preferably outdoors in natural light) are all attributed to improving systems across depressive disorders.

4. Get social

Isolation does nothing to improve SAD symptoms. Especially in winter it can seem appealing to retreat into your shell or stay at home on cold days and nights, but being around other people can boost your mood.

5. Stress management

During the winter, it can be hard for people with SAD to handle stress, which can worsen the condition. So do what you can to minimise stress. For example, try not to take up projects with an early spring deadline or even take on too much. Meditation and yoga can also help.

6. Medication and CBT

Where symptoms are worsening, doctors can prescribe anti-depressants to help you manage your SAD. Selective serotonin reuptake inhibitors (SSRIs) help by increasing your serotonin levels.

Cognitive behavioural therapy (CBT), a treatment often used for anxiety and depression, has been used as a treatment to stop SAD recurring in people year after year.

7. Make plans for spring

Having plans for spring, when the days will be longer and brighter, can often help you see the end of the winter coming, rather than it seeming to stretch on and on. This will give you something to look forward to.