According to Pamela Reid's "Dog In Sight":
Dogs have evolved from predatory ancestors and so, by evolutionary design, are crepuscular (more active at dawn and dusk). This is common in predators because prey species are more abundant-although more difficult to see- at dawn and dusk.
From Ian Dunbar's "Before and After Getting Your New Puppy"
Dogs are crepuscular and quite happy to sleep all day and all night. ...they have two activity peaks, at dawn and dusk.