The dangers of not getting enough sleep are widely known, but fewer people consider that sleeping too much may be—according to a growing body of research—also a risky endeavour.
For most adults, oversleeping counts as sleeping for longer than nine hours per night. (Somewhere between seven to nine hours is the recommended sweet spot).
Longer than that, and you may be at an increased risk for fatigue, lethargy, headaches, obesity, impaired immunity, impaired fertility, stroke, and general mortality, among other things.
If you think that sounds a lot like the problems typically associated with sleep deprivation, you’re right.
In general, researchers have picked up on a "U-shaped" relationship between sleep and increased mortality, where seven to nine hours is associated with the lowest risk while sleep durations at either extreme end up your risk.
The caveat to this research is that correlation does not equal causation—that is, just because people who oversleep seem to have an increased risk for mortality, doesn’t mean the former causes the latter.
In the case of sleep deprivation, it’s easier to point to causal mechanisms than with oversleeping.
There’s been a great deal more research on it, and studies about oversleeping rely more on self-reported sleep duration than those about sleep deprivation, which are more often performed in controlled environments.
With oversleeping, we’re faced with the chicken and egg problem.
Is it really the case that oversleeping causes health problems, or do people with certain health problems tend to sleep more?
For example, oversleeping is linked with fatigue and lethargy—but fatigue and lethargy may cause people to sleep for longer than normal.
Researchers have done their best to control for such risk factors and have proposed a number of ways oversleeping could mess with your health.
These include the fact that long sleepers tend to have more disturbed, fragmented sleep, less exposure to light, and changes to certain cells related to immune function.
However, the jury is largely still out. We know that there’s an association between oversleeping and increased mortality, as well as a variety of health problems, but we can’t say for sure in which direction the causal arrow points.
One thing is definite—if you habitually sleep for much longer than nine hours, it’s worth getting screened for underlying health conditions. Your doctor may recommend that you enrol in a sleep study, since you may be suffering from poor quality sleep overall.
While people’s individual sleep needs vary slightly, seven to nine hours is a solid general recommendation. Don’t neglect abnormalities at either end of the spectrum.