Scientists have a few theories about how too little sleep, or too much sleep, is associated with mortality, but we don’t have a handle on the big picture yet. The culprit for poor health for short sleepers may be sleep fragmentation, a common malady of multiple awakenings that prevents the body from getting enough restorative sleep (REM and Deep sleep).

Other physicians believe that the wrong amount of sleep weakens the immune system, so we’re more susceptible to other health conditions. Finally, for those who are long sleepers (8+ hours each night), the cause could be circadian disorders or simply a symptom of failing health.

Interestingly, several mortality studies have tested the common belief that 8 hours of sleep is best. A 2002 study that used data from over a million people found that those who sleep seven hours live the longest. This caused quite an outrage in the sleep medicine community, but the data is holding up to scrutiny.

 Lucky Number Seven:  Other studies have shown that those who sleep between 7 and 8 hours also have the lowest BMI (Body-Mass Index) and have the lowest risk of weight gain.

Relatedly, a 2011 study suggested that those who sleep less than six hours or more than eight may be aging the brain prematurely. For middle-aged adults, the perceived decline in mental abilities is equivalent to being four to seven years older.

That said, given how everyone has a unique relationship to sleep, it’s best to find the right amount that keeps you energetic and feeling good throughout the day… preferably in between seven and eight hours.