It is not just human beings, but also pets that suffer from bouts of depression during winter called Seasonal Affective Disorder (SAD), a survey has revealed. A survey conducted by leading veterinary charity PDSA found that one in three dog owners see a considerable downturn in their pets' moods during the winter months.

Half of the owners reported that they pets slept for longer and 20 pct of them said that their pets were lees active during these months. Thirty pct of cat owners also said that their pet seems "sadder" and less playful during the season.

"When considering our pets in this context, it's fair to say that the very poor summer we've had and the onset of dark nights, could certainly have an impact on our pets' mood," the Daily Mail quoted PDSA senior veterinary surgeon, Elaine Pendlebury , as saying.

"So, owners need to kick-start their pet's winter health routine with regular exercise and a well-balanced diet,” Pendlebury added. SAD is believe to be caused by low levels of light that cause a chemical imbalance in the part of the brain, which regulates sleep, appetite, mood and activity.

No, experts think that the same process can affect animals too. "The cause of SAD in people is thought to be linked to the levels of melatonin in the body," said Pendlebury.

"The production of this hormone is inhibited when light hits the eye's retina. Therefore more melatoninis produced when it is dark, which is why it is also called 'the hormone of darkness'.