For most of us, winter conjures up images of crisp, bright days and cosy nights snuggled round the fire, but for a significant number of people, the end of autumn signals the onset of a severe depression that will last until the arrival of spring. According to seasonal affective disorder (SAD) expert Dr Lance Workman, a biopsychologist at Bath Spa University, the cause is a biochemical imbalance in the hypothalamus of the brain, due to shorter daylight hours and lack of sunlight. "The official figures, which state that only 10 per cent of the population are affected, wildly underestimate the reaches of SAD," says Dr Workman. "It's a lethargic form of depression. Sufferers have no energy, low self-esteem, crave sugary foods and experience anhedonia - a lack of enjoyment in usually pleasurable activities. These SAD sufferers are often hyper in the summer months and the sort of people you'd describe as being ‘consistently happy'." "In evolutionary terms, our brains work this way so we can slow down in the winter when it's cold and there's less food around - it's a form of hibernation", explains Dr Workman. "But our needs and lifestyles have changed and many people are unable to keep up with the pace of modern life during the winter. The good news is now we know what causes it, we can successfully treat it. Ways to beat SAD 5-HTP Even if you just get the winter blues, you can boost your mood with your diet. The nutrient called tryptophan boosts production of the "happy hormone" serotonin. Try tryptophan-rich foods such as chicken, turkey, fresh tuna, tofu, soya, potatoes, bananas and dairy. Light therapy Getting out into the daylight in winter is vital! Find a way to get outside and absorb any light that you can. Be sure to take a multivitamin rich in vitamin D so that you can supplement the vitamin that you normally absorb from the sun!