This is not based on a tiny pilot study of 30 people. One of the largest sleep studies, involving one million volunteers over a period of six years, found that the optimal sleep time for longevity is sex to seven hours on average. Dr Daniel Kripke and his colleagues also have plenty of backup, as thirty out of thirty studies validate his findings. According to Dr Kripke, sleeping for more than seven hours, or less than five and a half, is associated with a shorter life. At the University of Massachusetts Medical School, Dr Gregg Jacobs has also found that five and a half hours of sleep is enough for optimal health. He has a concept called "core sleep".
spent 11 or more hours each day sitting down had a 12% higher risk of death, but lower levels of inactivity were not exempt from being harmful. Sitting for more than 6 or 8 hours a day is, as Dr JoAnn Manson says, not likely to be good for you. Fortunately, workplaces are beginning to adapt, with standing and treadmill desks as well as having nothing against you standing and stretching for a few minutes.
Maybe you sleep too much because you aren't getting "proper" sleep; in this case, there are simple things you can do to improve the quality and depth of sleep (which would also boost your motivation to exercise). One of these is "stimulus control", in other words, use the bedroom for sleeping and intimacy only. If you have been lying awake for more than half an hour in bed, leave the room and do something relaxing somewhere else. This was a main feature of Dr Jacob's Cognitive Behavioural Therapy for Insomnia program. His research on this involved ten thousand participants with a 90% success rate, and 85-90% were able to reduce or cut out sleep medications. Positive thoughts surrounding sleep, such as "It is fine to allow sleep to happen within 30 minutes of going to bed", and "It is natural to awaken briefly during the night since sleep is polyphasic", can also help. These can help us to un-learn the unrealistic expectations that the eight-hour belief taught us about sleep, which only increase stress and reduce sleep quality even more. Using both sleep scheduling, where you go to bed and wake up at roughly the same time every day, and sleeping in a dark, uncluttered room, can help to reset our 24-hour rhythms of rest and activity. While the right quality and quantity of sleep are very important, there's nothing to worry about, as you can start today with improving sleep hygiene.