Which is the problem being raised. According to the study 9 hrs of sleep is bad for your health (too much sleep). But that 9 hrs may actually constitute 8 hrs

Which is the problem being raised. According to the study 9 hrs of sleep is bad for your health (too much sleep). But that 9 hrs may actually constitute 8 hrs of actual physiological sleep. So 8 hrs of actual sleep is too much and is bad?