So how can you make yourself feel better and beat the winter blues? We have some top tips and expert advice from psychologist Donna Dawson and consultant to St John's Wort medicine Karma.
This is always the top treatment option suggested by experts. It's believed that SAD is caused by a lack of light that cuts serotonin in the brain - this is the chemical in the body that makes you feel happy.
You can buy a small lightbox and spend 30 minutes a day in front of it if you suffer regularly. The cost of these lights generally starts from £40-£50 and can go up to around £200, but they are usually a good long-term investment - people who've tried them often swear by their benefit.
If you can't afford a lightbox, then make sure you get outside more often. Donna Dawson is a psychologist specialising in personality and behaviour and recommends a daily dose of sunlight.
'Don't stay cooped up in your dark, over-heated, poorly-ventilated office or home. Natural sunlight can help to synchronize our body rhythms,' she advises.
'Without enough natural light, hormones can become unbalanced and cause mood disturbances, such as low feelings and depression. On sunny, or even cloud-covered days take a walk or eat your lunch on a park bench.'
Your diet often takes a nosedive during winter, because you crave comfort food, which is often rich in fat and sugar.
Experts suggest ditching the sugary snacks in particular because they make your blood sugar levels go up and down, which can have an impact on SAD symptoms.
Try to up the amount of oily fish, like mackerel, you eat - they are high in omega 3 oils and studies have shown that it can help improve symptoms of SAD.
Even better, mackerel and sardines are both a cheap option when it comes to fish, so it won't cost the earth.
You could also try brazil nuts, which are are high in selenium, a mineral that boosts production of feelgood hormone serotonin. A handful a day will give you your recommended daily allowance and also help boost your immune system.
St John's Wort
St John's Wort is a naturally-occurring herbal treatment that is also known as Hypericum Perforatum.
It has been used for many years to treat depression, and many experts recommend it for SAD because it has similar benefits to anti-depressants, without having to start on a course of medication.
However, you should still consult your doctor before you take St John's Wort, because it does have side effects and can interfere with some other medications like the contraceptive pill.
You can go and see your doctor and get them to refer you to a counsellor or therapist - talk therapy has been shown to help sufferers of SAD.
Alternatively, reach out to a friend. 'Let others know how you feel,' says Donna Dawson, 'and talk to them about your feelings and symptoms. A word of support or advice from a friend or loved one, a shoulder to lean on or a joke shared can help to lighten your symptoms.'
Exercise has been shown to help depression and SAD, so try to do something at least 3 times a week for 30 minutes.
Psychologist Donna Dawson says: 'Regular exercise is a great way to avoid the threat of low moods - it gets the blood flowing and releases feelgood endorphins, which are nature's instant mood lifters.'
SAD is frequently associated with tension, stress and anxiety. Yoga, reading, gardening or going away for a short holiday can help.
'Visualisation can be particularly effective,' says Donna Dawson. 'Take 5 minutes whenever possible just to escape to "your special place" in your head, whether it's a tropical island, mountaintop or garden. Use all your senses and see, hear, smell and feel it in minute detail.'
'Just a few minutes daily used in this way can keep stress from building up.'