How long do dogs live is a common question. The answer requires you to consider many factors, which include breed mix and size, and environmental influences. In general, smaller dogs live longer, from 14-22 years, with medium sized dogs living 11-14 years and large dogs living 8-12 years. Mixed breed dogs tend to live longer as many purebreds suffer from breed specific genetic problems that may shorten lifespan.
Diseases of the neurological system can also cause lethargy, and examples of these include fibromyalgia and Parkinson's disease. Diseases such as these have joint discomfort or stiffness, which may cause those affected to move less. Since they cannot move well without pain, this may increase their lethargy.
Lethargy is typically a symptom of a disease, and it has varied levels of medical seriousness. Overworking for extended periods of time is a common cause of lethargy, but it is far from the only one. Other causes of lethargy could be divided into different types.