Scottish scientists have identified how a hormone in a primitive type of sheep controls the animal’s seasonal behaviour.

They believe the origins of this mechanism could date back 500 million years and developed to ensure survival – including that of humans.

It is thought sufferers of Seasonal Affective Disorder (SAD), who have symptoms such as lethargy, depression and sleep problems in winter, may be highly sensitive to this seasonal trigger.

The research, published in the journal Current Biology, studied the primitive Soay sheep from St Kilda that survives harsh winter conditions.

Study leader Dr David Hazlerigg, reader in zoology at the University of Aberdeen, said: “We would speculate you find it across any seasonal mammal and it would probably turn out to apply across all different vertebrates, which means you go back 500 million years or more to its origins.

“These sorts of mechanisms might be involved when you see seasonal effects in humans and there is a possible link to SAD.”

These sorts of mechanisms might be involved when you see seasonal effects in humans and there is a possible link to SAD

Study leader Dr David Hazlerigg, reader in zoology at the University of Aberdeen

Although first recognised in the 19th century, SAD was only officially classified in 1984. It is estimated that it affects 500,000 people in the UK between September and April.