PhotoperiodEffect.com

17. But sleeping too much is supposed to be unhealthy. The studies these days say that seven and a half hours a night is ideal, any more or less than that and it's not good for you.

It's true that studies are showing that the healthiest people in industrialized countries sleep for around seven and one-half hours each night. Less sleep is clearly harmful, but those who sleep for longer periods of nine or ten hours are also less healthy. This may be simply be because we need more time to recuperate when we're ill, and therefore sleep longer (but would be even more ill if we slept less.) Then again, it may be that if we are sleeping with light on us, into the morning; that we tend to sleep more in order to make up for reduced levels of melatonin, yet still aren't getting enough melatonin because we aren't getting enough darkness.

So far, despite the striking effects of lack of sleep on many chronic conditions discovered in medical studies, and our current knowledge that critical nightly hormone rhythms are controlled by darkness; there has been no systematic study that would determine with more certainty whether too much sleep is actually a marker of insufficient darkness.