Unless it is a sudden change and accompanied by other symptoms, such as lack of appetite or other unusual signs, it is not an issue. Many breeds do not have high exercise requirements and few short walks and petting sessions is all they need. In addition, dogs that are home alone during the work week tend to sleep or rest during that time. Typically they will maintain their routine even when we are home on weekends, giving the appearance of sleeping too much. But it is their conditioned schedule, not an excessive amount of sleep.