Previously, it had been known that the body maintains daily cycles
call the circadian rhythm.
Now they have found the cells in the body that maintain annual cycles
which they call circannual rhythm.
These cells (about 17,000) are located in the pituitary gland,
which sits at the base of the brain,
and releases hormones that control processes throughout the body.
The cells have a "binary system" that can exist in one of two states.
They can either produce "winter" chemicals or "summer" ones .
More of the sleep hormone melatonin is produced in the winter when the days are darker.
Both the annual and daily body clocks are controlled by light
The annual pattern is used to trigger migrations, hibernations and mating seasons
and ultimately explains why lambs are born in spring.
It is still not clear how the body knows it is spring or autumn
when some calendar cells are in winter mode and others are in a summer state.
BBCNews - Body's 'chemical calendar' discovered