Both long and short sleepers die early. People who sleep a lot each night (long sleepers) and people with short slumber periods (short sleepers) tend to die younger than people with a more average sleep period. This a generalization of huge populations, so don’t necessarily worry if you are a long or short sleeper as individual differences swamp group differences. But it is an interesting finding that has been shown time and again in analysis of death statistics. Looking into the reasons may tell us something about the mysteries of sleep. A U-shaped mortality rate has been shown in many studies - with the lowest mortality rate at around 7 hours per night.
Epidemiologists like to use the metric relative risk when assessing mortality risks of behaviors. The relative risk is the ratio of the probability of death when the person has an abnormal sleep patterns to the probability when the person sleeps normally.
A large meta-analysis (study of other studies) found that short sleepers have a mortality risk ration of 1.10, while long sleepers have a mortality ration of 1.23.
Now the official cause of death is never (or almost never) a sleep disorder or insufficient sleep. The leading causes of death in the United States (as reported by the Centers for Disease Control) are heart disease and cancer.
For death by cancer, short sleepers have a relative risk of 0.99 (short sleepers are slightly less apt to die from cancer) and for death by cardiovascular disease, the RR is 1.06.
Long sleepers have a RR of 1.21 for cancer mortality and 1.38 for cardiovascular mortality. People who sleep more than 8 hours per night also have a higher incidence of metabolic syndrome than those who sleep between 7 and 8 hours.
The concept of allostatic load, although not firmly embraced by academic medicine, may come into play here.
Diffuse and subtle assaults on the various organs and systems of the body are sometimes summed up into the concept allostatic load: – the wear on the body and the chronic response by the immune system. Many systems in the body are set up to maintain some regularity – homeostasis – and generally these work well. But to stay on course - to maintain the homeostasis – the body has to apply resources and over time this constant resetting and effort to stay on course leads to "wear and tear" on the body. Whether allostatic load is a real thing is not clear – it hasn’t been totally embraced by the medical establishment – but it plays into plausible hypotheses about causes of long-term "old person" diseases such as metabolic syndrome, diabetes, cognitive decline, and other undefined signs and symptoms of old age.
Could the allostatic load from years of short sleep increase the odds of fatal illnesses? That’s an idea floating around sleep research circles.
So that might be an explanation for why short sleepers die prematurely, but it doesn’t ring true as a reason for why long sleepers have a higher risk ratio. A more plausible reason might be that the long sleeper has other, perhaps undiagnosed, problems that cause the person to sleep a lot. If the long sleep is classified as hypersomnia, these other problems could be classified as comorbidities.
Now what happens when a short sleeper has the symptoms of insomnia? Some people are short sleepers but feel fine during the day and thus do not meet the diagnostic criteria for insomnia. A Penn State study of such people (chronic insomniacs with short sleep) found males had a higher mortality than normal sleepers. Females with this condition had only a slight increase in mortality. This may be indicative of the higher percentage of men who get cardiovascular problems in middle age than women.
If we ignore the allostatic load concept and just consider visible mechanistic causes, we can see several possible reasons short sleep increases mortality: