A study of wild sheep in the Western Isles has uncovered secrets of the body's seasonal clock, according to experts.
Sad can damage quality of life
The Medical Research Council (MRC) in Scotland said the work had revealed genetic clues to changes affecting mood, appetite and energy levels.
It is hoped the work will help in the treatment of Seasonal Affective Disorder (Sad).
Sad affects about 500,000 people a year in the UK, between September and April.
The sheep study findings have been published in research journal Current Biology.
The MRC human reproductive sciences unit studied soay sheep on the remote islands of St Kilda, along with colleagues from the University of Aberdeen.
Work on the breed, which is said to have highly seasonal reproduction, discovered that the body's seasonal clockwork is dependent on processes occurring inside individual brain cells.
The process was found to be driven by the way certain genes interact with each other, and this in turn was affected by day length.
Lead scientist Dr Gerald Lincoln said the changes at a molecular level could soon help identify the causes of Sad.
He said: "Because our moods, appetites and energy levels are affected by the seasons, unravelling at a molecular level what makes our seasonal clocks tick is important for understanding and treating seasonal and lifestyle related illnesses and diseases.
"We would like to understand the reasons why people suffer and there's a huge variation amongst people in how much they are affected.
Soay sheep have been the focus of previous research
"When we get to know more about it we'll actually be able to understand why people do differ as they do in their response to the seasons."
The study concluded that night length affected the timing of the clock genes through the production of a hormone called melatonin.
Melatonin is produced when it is dark by the pineal gland in the brain.
It is hoped the work will lead to more effective treatments of the disorder, which can be combated with high doses of artificial light.
Soay sheep are thought to date back to Roman times and one of their largest wild populations is found on Hirta, in the St Kilda group, which was evacuated of islanders in the 1930s.
An earlier study of the breed revealed links between male aggression and sudden drops in the hormone testosterone.