Apparently, we get SAD as well!
This is what I've found on one of the doggy websites.
SAD' Threatens to Take Over Pet Population [30/10/2007] A recent study by the PDSA found that as the days get shorter pets could be suffering from Seasonal Affective Disorder (SAD). The symptoms of the disorder include eating more, sleeping longer and being less active.
The results of the study found that two in five dog owners felt their dog became less active over the winter months whilst one in three cat owners felt their pet became “sadder” and less interested in playing.
Senior Veterinary Surgeon, Elaine Pendlebury commented “The cause of SAD in people is thought to be linked to the levels of melatonin in the body. The production of this hormone is inhibited when light hits the eye's retina. Therefore more melatonin is produced when it is dark, which is why it is also called 'the hormone of darkness'. When this hormone is released into the bloodstream it is thought to make us drowsy and a little down in the dumps.”
She added: "Of course, the change in our pet's mood may be simply due to the cold weather and fewer opportunities to stretch their legs outside, but diet and exercise can play a big part in perking up your pet. Even if you can't go out, there are various indoor games that you can play with your pet.
