If you're feeling sadder as the days get shorter, you're not alone. Around half the population report feeling less happy in winter and 5 percent fall into serious depression.

Apart from nutrition and exercise, getting more light in your eyes is the best treatment we know for seasonal mood challenges, says NZ-born neurobiology researcher Anna Wirz-Justice, who is based in Basel, Switzerland, where she is emeritus professor of psychiatric neurobiology at the Centre for Chronobiology.

"The clinical studies that have used light as therapy have shown this is a very powerful treatment," she tells Susie Ferguson on Saturday Morning.

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We don't yet know why some people are more vulnerable to seasonal mood challenges, Wirz-Justice says, but it may relate to serotonin levels and depression vulnerability in general.

The psychological condition known as Seasonal Affective Disorder or SAD is found all over the world but the latitude of where you live can add to your risk.

As New Zealand is a "long and thin" country, the amount of winter daylight on offer varies a lot, Wirz-Justice says.

Those living in the "middle latitudes" - including in Wellington and Christchurch - are particularly vulnerable to getting down as daylight diminishes.

For the large number of people prone to falling into the "winter blues", more exposure to sunlight can be all that's needed to keep depression at bay, she says.

"Light - and in particular daylight, which we don't often get out into as much as we should - is so important for our basic health, for our psychological health, for our physical health."

Professor Anna Wirz-Justice. Photo: The Daylight Award

For the 5 percent of those who experience a "major depressive disorder" every autumn and winter that spontaneously lifts in spring and summer, the therapeutic use of bright artificial light is the best treatment we currently have, Wirz-Justice says.

Sitting in front of intense light from a special light box for half an hour a day, preferably in the morning, can be effective in lifting the mood of these people, she says. 

These light boxes have an intensity of about 5,000 to 10,000 LUX (a measurement of light on a surface), Wirz-Justice says, which is equivalent to the light on "a bright, sunny day in the early morning".

Indoors, the light intensity we are exposed to - usually around 50 to 500 LUX - is not strong enough to stabilise our biological clocks, she says.

Outdoors - even on a grey, rainy day - the light is much more intense.

To keep the winter blues at bay, Wirz-Justice recommends people get outside for half an hour every day, preferably in the morning.

In the open air, you will get at least 3,000 LUX, she says, which is enough to synchronise your body clock for the day.

As the positive effects of light on our biological rhythms occur through eye contact, it's best not to wear sunglasses during this morning light fix.

"We are not talking about light on the skin. We're talking about light hitting the retina and going straight to the brain to affect cognition and mood and rhythms and sleep - and that's all through the eyes."

Exploring the vast potential of sunlight for human and planetary well-being is the focus of Daylight Academy - a group of researchers, scientists and designers of which Wirz-Justice is part.

Increasingly, hospitals are being designed with indoor light in mind, she says, and dementia care facilities with lighting systems that artificially signal dawn and dusk.  

"The ramifications of what we know about how light affects human behaviour and physiology and psychology is growing and is enormous."