Sunlight continues to cause damage which can trigger skin cancer long after sun worshipers have moved into the shade, a study by Yale University has found.

Although many people move indoors before they feel the tell-tale sting of a sunburn, harm is still occurring for three hours afterwards.

US scientists have suggested that "evening after" sunscreen containing vitamin E should be applied at night, or after tanning sessions, to prevent the ongoing dangers UV radiation.

The source of the hazard was found to be melanin, the tanning pigment that normally shields the skin from too much sunshine.

Professor Douglas Brash, from Yale University, said: "If you look inside adult skin, melanin does protect against (DNA damage). It does act as a shield. But it is doing both good and bad things."

The research shows that ultra-violet (UV) radiation from the sun generates highly reactive forms of oxygen and nitrogen in the skin that excite electrons in melanin.

This process, known as chemiexcitation, can lead to breaks in DNA that can occur even in darkness up to three hours after sun exposure and raise the risk of skin cancer.

Previously, chemiexcitation was only known to occur in plants and lower forms for animal life.

The finding means that melanin both protects against damage from sunlight and contributes to carcinogenisis.

The US researchers focused on a type of DNA damage known as a cyclobutane dimer (CPD) in which the genetic code is prevented from being read correctly.

Alarmingly it appear that half of the damage done by sunlight actually occurs after exposure has ended.

However they found that vitamin E offered a potential way of combating the effect, since it both acted as an antioxidant - having the ability to suppress reactive oxygen - and blocked the energy transfer involving excited electrons that disrupted DNA.

"One benefit of dark photochemistrys slow course is that it allows intervention," wrote the authors

"Screening ... offers the prospect of developing 'evening-after' sunscreens that could potentially prevent the carcinogenic processes occurring in the skin hours after sunlight exposure ends."

The researchers, who studied the effect in mouse and human cells, report their results in the journal Science.