Studies in the Elderly
In general, studies that have focused on elderly populations have supported findings from earlier research. Branch and Jette10
observed findings similar to the Alameda County Study. Among the elderly, both short (<7 hours) and long (>8 hours) sleepers were at increased mortality risk. Another study in a sample of nursing home residents found that age-adjusted mortality rates were highest in women sleeping ≤4 hours and ≥9 hours and men sleeping ≤5 hours and ≥9 hours11
. However, these risks became non-significant after adjusting for covariates. In a study of the elderly in the UK, significant increased mortality risk was found for those reporting ≥10 hours of sleep, and for those who reported “insomnia often.12
” However, this study did not account for gender and the reference group had a very broad range of sleep duration (4.0–9.9 hours). Gale and Martyn13
examined data from adults aged ≥65 years in the UK and found that, compared to 9-hour sleepers, those sleeping ≥10 hours were at increased risk of mortality. Conversely, analyses of health survey data from Spain (Barcelona) did not find significantly increased risk in elderly short or long sleepers, compared to the reference group (7–9 hours), though a non-significant increase for long sleep was observed14
While there is some variation, taken together, these results reveal that in the elderly, increased mortality is associated with both short and long sleep.
Sleep and Mortality: A Global Issue
Following earlier studies, scientists have explored this association around the world. Several of these studies occurred in Japan15–19
. One study found increased mortality risk in those reporting sleep ≥9 hours, as compared to 7–8 hours in the farming town of Wakuya15
. An analysis of the general population of Gifu prefecture aged 20–67 found that in age-adjusted analyses, increased mortality risk was associated with short (<7 hours) and long ( 10 hours) sleep compared to the reference group (7.0–8.9 hours), but after adjusting for covariates, only the short sleep association remained significant16
. In a study of elderly residents from the village of Ohgimi, increased mortality was seen among those sleeping <6 and >7 hours in a model adjusted for age and health behaviors, but when physiological measures of health and functional status were entered into the model, only the relationship with short sleep remained statistically significant17
More recently, two large-scale studies have examined these relationships in samples representative of the general population of Japan. In one study18
, compared to the reference group (7.0–7.9 hours), men were at increased risk of mortality if they reported <6 hours of sleep, and women were at increased risk if they reported ≥9 hours. The largest sleep duration and mortality study in Japan investigated over 100,000 adults from 45 areas of Japan19
. Compared to the reference group (7.0–7.9 hours), both men and women were at increased risk of mortality of they slept 8.0–8.9 hours or ≥9 hours on average. Further analysis of this dataset20
reported that in men, ≥9 hours of sleep was associated with increased risks of all-cause mortality, cardiovascular mortality, esophageal cancer, and pancreatic cancer, but decreased risk of prostate cancer; <7 hours of sleep was associated with a decreased risk of stomach cancer. In women, ≥9 hours of sleep was associated with increased all-cause mortality, cardiovascular mortality, urothelial cancer and non-Hodgkins lymphoma; <7 hours of sleep was associated with a decreased risk of lung cancer and cardiovascular disease.
A 10-year study of adults over 60 in Taiwan21
found increased mortality risk (relative to 7 hours) associated with ≥10 hours of sleep in men, and all categories ≥8.0 hours of sleep in women. Analysis of the Singapore Chinese Health Study of ethnic Cantonese and Hokkien adults aged 45–74 found significant increases in mortality risk (relative to 7 hours) associated with ≤5, 6, 8, and ≥9 hours, though after adjusting for covariates, only the relationships in the most extreme categories remained significant22
In Israel, Burazeri and colleagues23
examined data from adults ≥50 years old. Adjusted analyses found that in men, increased mortality risk was associated with sleep >8 hours, compared to a 6hr reference group. For women, mortality risk was lower for those reporting 6–8 hours of 24-hour sleep, compared to those sleeping <6 hours.
In Sweden, an analysis of survey data from adults aged 45–65 years found that age-adjusted mortality risk was increased in male long sleepers (>8 hours) compared to the reference group (6–8 hours), though this result became non-significant after adjusting for all covariates24
. No relationships were seen for women.
In Finland, sleep duration data for adults (age 24–101) comprising all twins born before 1958 was analyzed25
. When compared to the reference group (7–8 hours), increased mortality risk was found for men and women who reported <7 or >8 hours of sleep.
In the United Kingdom, three studies have investigated non-elderly, general population samples26–28
. Huppert and Whittington26
found that sleep duration was not related to mortality in women, but long sleep duration (>8 hours) predicted mortality in men. Heslop and colleagues27
examined a sample representing the general population of Glasgow, Scotland. This study investigated sleep duration at multiple times. Women reporting <7 hours at second screening were at increased risk of all-cause and cardiovascular mortality when adjusting for covariates, but this relationship became non-significant when stress level was introduced in the model. For men, age-adjusted analyses showed increased risk in those who were <7hr sleepers at both time points compared to those 7–8hr sleepers at both time points, but this became non-significant when marital status and social class were included. Men and women who were always <7hr sleepers demonstrated increased risk for all-cause mortality in models adjusting for age, marital status, social class and health risk factors. Inclusion of stress level significantly attenuated the short sleep-mortality association to the null.
Other studies have also measured sleep duration at multiple points in time. Ferrie and colleagues28
longitudinally examined mortality risk for sleep durations measured during two time periods. The data came from the ongoing Whitehall II cohort. For the first time period, only ≤5 hours of sleep was associated with increased all-cause and cardiovascular mortality risk in adjusted analyses, and only in models that adjusted only for age. For the second time period, increased all-cause mortality was associated with ≤5 or ≥9 hours of sleep and increased cardiovascular mortality was associated with ≤5 hours of sleep. Those reporting having slept 7–8 hours at baseline were protected if they remained in that category at the second time period. Interestingly, mortality risk increased if they deviated above or below that duration category. Those reporting 6 hours of sleep benefitted from an increase to 7 hours, but not 8 hours. Subjects reporting ≤5 hours were subject to decreased mortality risk if they reported 6 hours at the second time point.
Finally, there have been several studies in U.S. population samples29–35
. Qureshi and colleagues29
examined data from the National Health and Nutrition Examination Survey (NHANES) and found an increased risk of mortality in long sleepers (>8 hours). Chen and Foley found that mortality was associated with <6 or >9 hours of sleep, but only in those over 60 years of age36
. A recent reanalysis of the NHANES data35
found that even after controlling for a variety of covariates, short sleep of ≤5 hours, as well as longer sleep of 8 and ≥9 hours, was associated with increased mortality, relative to 7 hours. However, analysis of age sub-groups (<60 versus >60 years), revealed that only subjects aged >60 demonstrated showed a significant mortality relationship with short and long sleep duration.
Data from the well-known cohorts of the Framingham Study30
and Nurses’ Health Study32, 34
also support the elevated mortality risks associated with short and long sleep. The former study found the highest mortality risk in subjects reporting <6 hours and >9 hours. The latter reported increased all-cause age-adjusted mortality risk (relative to the 7 hours) in all short (≤5 and 6 hours) and long (8 and ≥9) groups.34
Ayas and colleagues32
also examined this dataset in another publication and found similar results.
Dew and colleagues33
reviewed data from several studies to conduct the first electroencephalographic study of sleep and mortality risk. In their sample of 184 individuals, short sleepers (<6 hours) did not have significantly higher mortality risks than the rest of the sample. However, increased mortality risk was associated with sleep latency >30 minutes, sleep efficiency <80%, and slow wave sleep <1%.
Kripke and colleagues conducted the largest study of sleep duration and mortality using over 1 million records from the Cancer Prevention Study II31
. After adjustment for demographic risk factors, health habits, status, history, and medications, they found distinct U-shaped curves across sleep duration categories, demonstrating increased mortality risk the further the deviation from 7 hours. Thus, those who reported sleep of 7 hours (6.5–7.4 hours before coding) were at the lowest risk of mortality and those sleeping <6.5 or >7.4 hours were at increasingly greater risk, the further from the nadir of the U curve. Depression and stress were not included in these analyses.
Limitations of previous epidemiological studies
Despite the valuable contributions of prior research, there are several limitations of this literature. One important limitation of prior epidemiological studies is that definitions of “short” and “long” sleep vary across studies, thus preventing adequate comparisons. For example, while the study by Kripke and colleagues31
contained groups representing every hour of duration within the sample, the study by Rumble and Morgan12
measured only three groups, with the reference group reporting 4–9.9 hours of sleep. Additionally, while the various survey questions used in these studies may have face validity, it is unclear that they are reliable and valid measures of sleep duration. These surveys employed questions which have not been validated against objective sleep recordings; thus, it is unclear what precisely they are measuring. It may be the case that factors such as time in bed, demand characteristics, social desirability, paradoxical insomnia (i.e., sleep state misperception), and other factors covary with these self reports of sleep duration.
Another important source of heterogeneity is the inclusion of covariates. Many studies found relationships only when adjusting for age and in most cases, gender. This suggests that there is an age (or cohort) effect, and a difference between men and women. Other covariates, including indicators of health status and history, sociodemographic and socioeconomic factors, functioning, medication use, and psychological morbidity, varied between studies, with a range of 03, 7-3231 covariates besides age. Both under adjustment and over adjustment present concerns. Not adjusting for enough variables could mean that psome relationships are driven by third factors, and not by sleep duration. Over adjusting may remove some of the causal effects of sleep, if the covariate is on the causal pathway between sleep and mortality.
Also, the stability of sleep as a trait in relation to health outcomes has not been established. The studies that only measure sleep at one time point assume that self-reported sleep duration at one time represents a stable exposure.
Finally, the majority of studies reviewed in this paper use datasets that were not designed to evaluate relationships with sleep. Frequently, analyses are constrained to use the only sleep question available in the survey (usually an estimate of habitual sleep duration), which limits the reliability and validity of the findings.