When a sleep disorder becomes chronic it can be associated with type 2 diabetes, hypertension, obesity, metabolic syndrome, GI symptoms, stroke and heart disease.
Not only do conditions like sleep apnea or sleep deprivation/insomnia cause fatigue, this could be associated with physical and behavioral changes as well. Ultimately the immune system will also become affected.
Ultimately effective treatment of your sleep disorder or sleep apnea will ultimately improve your chronic disease or internal medicine problems.