Sleep duration: risk factor or risk marker for ill-health?

Epidemiologists and non-epidemiologists in many parts of the world are now investigating whether variability in human sleep duration is a determinant of various diseases. But whilst responses to asking somebody how much they sleep usually appears to be a risk marker for disease, is it really the factor that causes disease (a risk factor) or is the real causative agent causing us to observe an erroneous association (confounding)? This issue is not merely academic, as alleviation of the supposed problem hinges vitally on whether we are observing a risk factor or a risk marker/confounder. Treatment to improve a risk marker will be an expensive waste of time because it is not the real cause of the disease, merely a correlate.