How much sunshine do we really need? - BBC

Photographs of Coco Chanel returning bronzed after a Mediterranean cruise in 1923 popularised the idea of tanning and sunbathing. Her tan became a status symbol and transformed Western attitudes to sunshine.

From the early to the mid-1900s, heliotherapy – the therapeutic use of sunlight – was used to treat rickets and tuberculosis in sanatoriums, and a "healthy tan" was actively encouraged.

A century later, our relationship with the Sun has become much more complicated. In the past 50 years, increased skin cancer rates have led to a medical rethink, and public health campaigns have shifted towards avoidance and heavily promoted the use of sunscreen. The evidence linking excessive sun exposure with skin cancer risk still holds up today.

But in recent years, so-called "moderate" sun exposure – time-limited bursts of exposure, in the middle of the day – has been found to be beneficial for our mental health, vitamin D levels and immune system. Studies also show it reduces the risk of certain cancers, boosts cardiovascular health, and improves cognition and sleep. So, how can we reap the benefits while avoiding the dangers? And how much sunshine do we really need?

Why do we need sunshine?

Moderate sun exposure helps the body produce vitamin D, which is essential for bone health and muscle function. Some studies show that high vitamin D levels are linked to lower cancer incidence and mortality, as well as a reduced risk of developing type 2 diabetes.

Sunshine also lowers blood pressure. When UVA rays hit our skin, they trigger the release of nitric oxide, which causes our blood vessels to dilate. This reduces the risk of heart attacks and strokes.

A landmark study of 29,518 Swedish women over 20 years found that sunshine avoiders had double the mortality rate (from all causes) compared with the highest sun exposure group. Conversely, sunbed use correlated with an increased overall mortality and cancer mortality rate. Another Swedish study conducted in 2011 found that women who used a sunbed once a month or more for at least a decade had a 90% increase in all-cause mortality.

Rebecca Mason, a professor of physiology at the University of Sydney in Australia, says that getting adequate vitamin D levels helps to " dampen inflammation, respond to invading pathogens, and manage your immune system so it doesn't attack your own cells".

Sunshine has been shown to improve sleep and boost cognitive health. Morning sunlight contains blue light wavelengths that trigger your brain into thinking it's daytime, increasing cortisol and suppressing the sleepy hormone melatonin. This leaves us feeling more alert and energised. Higher exposure to daytime light can improve cognitive performance, reaction times and memory. In one study involving over 360,000 participants, daytime sunlight exposure (specifically around 1.5 hours) was linked to a reduced risk of developing dementia.

Sunlight exposure also increases the level of the chemical messenger serotonin in the brain, which help us to feel calmer and more focused. Patients treated for severe depression in sunny rooms have been shown to recover more quickly. Meanwhile, insufficient exposure to daylight can increase the risk of depressive symptoms, low mood and insomnia.

How much sunshine do we need?

While guidance varies according to age, geography and skin type, experts typically recommend short, frequent bursts of sunshine.

Mason says that, in terms of vitamin D synthesis, exposure is most effective in the "middle of the day, little and often", when UVB levels are at their highest.

For people with light skin, Mason recommends regular, short bursts of sunshine on most days of the week, with the hands, arms or legs uncovered for a few minutes at a time. In very high UV areas, this exposure should be taken in the mid-morning or mid-afternoon instead.

Those with dark skin may need significantly more sun exposure, because the higher levels of melanin also mean that vitamin D production from the Sun takes longer. One study found that in the spring and summer in UK, those with dark brown skin need around 25 minutes of sun exposure at midday to produce adequate vitamin D.

The UV Index rates the total amount of UV radiation (including both UVA and UVB) which reaches the ground, measuring it on a scale of 0 (no light) to 11+ (extremely high radiation).