A team analysed the brains of sheep at different times of the year.

They found a cluster of 17,000 "calendar cells" in the pituitary gland, which sits at the base of the brain, and releases hormones that control processes throughout the body.

The research team say the cells have a "binary system" like a computer that can exist in one of two places - they can either produce "winter" chemicals or "summer" ones.

However, it is still not clear how the body knows it is spring or autumn when some calendar cells are in winter.

This annual clock is known as the circannual rhythm and is the longer-term cousin of the circadian or daily rhythm which keeps us awake at the right time of day.

The annual pattern is used to trigger migrations, hibernations and mating seasons and ultimately explains why lambs are born in spring.

Both the daily and annual body clocks are controlled by light.

More of the sleep hormone melatonin is produced in the winter when the days are darker.

Prof Loudon said: "We've known for some time that melatonin is critical for these long-term rhythms, but how it works and where it works had not been clear until now."

Prof Dave Burt, added: "The seasonal clock found in sheep is likely to be the same in all vertebrates, or at least contains the same parts.

"The next step is to understand how our cells record the passage of time."

Even though people do not have a mating season, there are signs we are still influenced by the seasons.

A study earlier this year, showed human genes involved with immunity became more active in the cold.

They said it could help fight off winter viruses such as flu but may make some conditions like arthritis worse.


Source :BBC