As I have understood from reading stuff on the internet and talking to people, too little sleep can have very serious health problems and you need to sleep at least 6 hours every night. what I don't know is if there is an upper limit to the the amount of sleep you get. Can sleeping too much also have detrimental health effects? What is the maximum amount that someone should be having every night?
People who sleep less than 6 hours and people who sleep more than 9 hours have been found to have a greater risk of suffering from adverse health effects, see e.g. here. But one cannot conclude from such observed correlations that changing sleeping behavior will help, this requires one to analyze the cause of these correlations. The way long sleep is associated with increased mortality is not well understood, as mentioned in this article:
It is therefore possible that long duration of sleep might be a consequence of, rather than a causative risk factor for, unrecognized chronic comorbidity, which in turn could explain the higher risk of mortality, particularly mortality from noncardiovascular causes, observed in many studies (1–4). Long sleep duration might represent a useful diagnostic tool for detecting other subclinical or undiagnosed mental or physical comorbidity (13).