Sleep and Mortality

^{June 2020}

For over 50 years, doctors and scientists have recognized “the critical importance of sleep to good health and life.”¹ Though initial studies focused on sleep deprivation, the full body of research now shows adverse health impacts for both overly short and long sleep. Atypical sleep duration is associated with adverse medical conditions, such as cardiovascular disease, obesity and physiological stress, along with resultant higher mortality; age, gender and socioeconomic status mediates these relationships.²

We analyzed the effectiveness of daily sleep duration in stratifying the mortality risk profile of a U.S. insured population simulated from National Center for Health Statistics (NCHS) survey data.

Data and Extrapolation to Life Insurance

The data analyzed was derived from the National Health and Nutrition Examination Survey (NHANES) conducted by NCHS, a program of studies designed to assess the health and nutritional status of adults and children in the United States. The survey examines a nationally representative sample of about 5,000 persons each year, and sampling weights are provided to ensure the weighted results are indicative of the country as a whole.³

Data was limited to survey years when sleep information was collected, which spans 2005 to 2014. Mortality information, which NCHS sources from the Social Security Administration and Centers for Medicare and Medicaid Services, is available through December 31, 2015, with an average follow-up time of six years in the final sample. Fields available in the data include:

• Demographic information including age, gender and household income
• Medical examination and laboratory data including BMI, blood pressure and cholesterol
• Self-disclosed smoking status and history of cancer, stroke, and diabetes
• Self-disclosed typical hours of sleep on weekdays/workdays
• Vital status of each individual and, if relevant, the date at which death was observed and high-level underlying cause of death categorization

Using this data, Munich Re simulated an insurable population by selecting lives with:

• Age 18 to 84
• Household income greater than $20,000
• No history of cardiovascular disorder
• No history of cancer (other than non-melanoma skin cancer)
• BMI within insurable limits (16 – 48)

The insurable population consists of 19,037 lives and 800 deaths. Each life is weighted by the interview weights provided by NHANES so that the final dataset is roughly equivalent to an insured population drawn from the U.S. population at the midpoint of the survey period.³

Key Findings

• Self-reported hours of weekday sleep effectively stratifies mortality risk.
• Individuals with atypically short or long daily sleep duration have elevated mortality, controlling for gender, age, and smoking status.
• Relative mortality for long sleep (10 hours or more) is approximately 2-3 times greater than for 7 hours of sleep, which is the population median and generally recommended amount of sleep.
• The adverse impact of long sleep duration on mortality is greater for older individuals than younger individuals, whereas the adverse impact of short sleep duration is greater for younger individuals than older individuals.

Overall Results

Figure 1a shows that the bulk of individuals sleep six-eight hours each night, with a median of 7 hours, which aligns with the generally recommended amount of sleep for an adult. However, there is still a significant population (15%+) with short or long daily sleep.

Figure 1b demonstrates that hours of weekday sleep stratify mortality risk, with both the “short sleepers” and “long sleepers” showing significantly higher mortality, making a characteristic U-shape in the one-way A/E plot. Short sleep duration (under 6 hours a night) and long sleep duration (more than 8 hours) is a risk factor for weight gain, hypertension, cardiovascular disease, and Type 2 diabetes.⁵

By Age

Although patterns of sleep vary throughout a person’s lifetime, reparative sleep needs remain relatively constant throughout adulthood. Older individuals tend to report more sleep interruptions or difficulty falling asleep, related to changes such as shift in circadian rhythms, increase in sleep disorders or other illnesses, and use of medications that disturb sleep.⁶ Figure 2a shows the distribution of daily sleep distribution by age, with individuals under the age of 30 dropped due to lack of credible death data for these younger ages. The oldest age range, 70-84, experiences longer sleep duration than younger ages, which is slightly counterintuitive to the expectation that older individuals would have more medical conditions that disturb sleep. It is possible that the elimination of cardiovascular disorders and cancer in the insurable population simulation may be partially responsible for this reversal.

The A/E bars in Figure 2b illustrate that short sleep duration can be particularly harmful for ages 30-49, though there is also somewhat limited death data for these younger ages. Excessive hours of sleep is harmful across all age groups compared to the baseline of seven hours.

Cause of Death

As discussed above, abnormal sleep duration is a risk factor for cardiometabolic risks, including heart disease, so we expected to see an association between actual causes of death related to heart disease and self-reported sleep hours. High level cause of death groupings are provided in the NHANES survey results, summarized as the three cause of death categories shown in Figure 5. Figure 5 demonstrates that individuals self-reporting either short or long sleep duration have an increased proportion of deaths attributable to heart disease or stroke.

One caveat is that “voluntary” short sleep (such as an individual spending less time in bed) versus short sleep due to an underlying disorder (such as insomnia or obstructive sleep apnea) cannot be distinguished in self-reported sleep, and these two scenarios likely exhibit different pathways for the impact of sleep duration on health and mortality.⁵ However, if the ultimate outcome is higher risk in either scenario, then sleep duration is still an appropriate segregator even if the exact mechanism is unknown. Relationships between sleep and mortality are also mediated by socioeconomic status so trends may be attenuated when indicators of socioeconomic status are included in the analysis.