It depends on the individual. Six hours is around the short extreme, and nine hours is around the long extreme. Most people fall somewhere in the middle. How can you tell what’s right for you? One clue is whether you feel work or family pressures are preventing you from going to bed, or staying in bed, when you’re sleepy. The best measure, though, is if you feel alert throughout the day, stay in a reasonably good mood, and don’t oversleep on days off. If you don’t do all three of these things, you should test whether a longer sleep interval — daily throughout the week — does the trick. On the other end, some people allow themselves to sleep longer than they need, and they pay for this with low mood and fatigue during the day. They often say to themselves, “If I could only sleep longer, I would have more energy the next day.” That may be a reasonable intuition, but it is a mistake. Longer sleep than necessary can feed depression and a sense of exhaustion.