Dogs sleep more than us, but they wake more frequently than we do. How much and when they sleep depends on the level of activity in their environment. A dog living as a pet in the home is likely to sleep more than a dog that works for a living, like a search and rescue dog or a dog working on a farm.
In the colder winter months, some rattlesnake species enter a period of brumation, which is dormancy similar to hibernation. They often gather together for brumation in large numbers (sometimes over 1,000 snakes), huddling together inside underground rattlesnake dens or hibernacula.
In the Judeo-Christian tradition, snakes are associated with Adeam and Eve and the Garden of Eden. They represent all that is evil, corrupt and dangerous. Others say that snakes are a sign of sexuality, potency or psychic energy. In your dream it sounds like your association to the snake was one of evil and mistrust.