Scientists have discovered a cluster of cells within the body which act as a kind of "chemical calendar". Some 17,000 or so cells reside in the pituitary gland that have the ability to release hormones that pass through the body and create a "binary system" that produce "winter" or "summer" chemicals, a team from the Universities of Manchester and Edinburgh revealed.

Published in Current Biology, the report states that the cells rely on the length of the day – shorter in winter, longer in summer – and have the capability in animals to alert them when to hibernate and breed. Based on the analysis the team did on sheep, the teams said this explains why lambs are typically born in spring.

The scientists say that this may alter the immune system in humans, which typically needs to be stronger in the colder months. Additionally, they are used for migration purposes in animals and although humans do not have a mating season, it is thought that the cells do influence this in us. Although it is unclear how the body knows what season it is in, the proportion of said cells, known as the circannual clock, alter to mark a full year.

Professor Andrew Loudon from the University of Manchester told the BBC: "It looks like there's a short period of the year in the middle of winter and the middle of summer when they are all in one state or the other. "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, from the University of Edinburgh, 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."