New season trends?

How do the seasons affect dogs?
Does their behaviour change depending on the season? Humans can get SAD (Seasonal affective disorder) where we feel more depressed when the days are longer, but can dogs suffer from a similar thing? And does day length affect them?

Anon, via email.

Down in the dumps

Can dogs suffer from Seasonal Affective Disorder (SAD)? For the past couple of weeks both my Labradors seem to have lost their 'spark' - even the usually much-anticipated visit from the postman evokes nothing more than a whimper. Both are sleeping more than usual too. Do you think they are depressed?

Jane Kerry, by email