U-Shaped Curve Describes the Link Between Sleep Duration and Mortality

A recent study published in the journal Sleep (vol. 30; No. 10, 2007, p.1245) examined the relationship between sleep and risk of mortality in over 21,000 Finnish twins. The study was conducted by Dr. Christer Hublin, MD, PhD and his colleagues at the Finnish Institute of Occupational Health, Helsinki, Finland, where they studied a large population-based cohort of Finnish adults.

The authors concluded that sleeping less than 7 hours and sleeping more than 8 hours dramatically increased the risk of mortality. Previous research has shown a U-Shaped curve to describe the relationship between sleep duration and mortality, where those who sleep 7 hours on average have the lowest mortality rates.

Questionnaire Data Collected from 21,268 Twins

The data collection began in 1974 and repeated in 1981 includes questionnaire data with information about sleep duration and sleep quality (N = 21,268; 52.3% female / 47.7% male; mean age 40.7 ± 13.5 years). Subjects were 21,268 twins.

The authors categorized sleep length into 3 categories; Short Duration (<7 hours), Average Duration (7-8 hours) and Long Duration (>8 hours). They also categorized Sleep Quality by the self reports of Sleeping Well, Sleeping Fairly Well, and Sleeping Fairly Poorly/Poorly. Finally, the authors categorized the use of sleep promoting medications by Not at all, Infrequent Use (1-59 days/year), and Frequent Use (>60 days/year).

Follow up-data was obtained from the Finnish national registry in 2003, by analyzing whether subjects were still living or had moved away from Finland.

Seven to Eight Hours Optimal

The authors concluded their results to support the U-Shaped association between sleep length and mortality, with the risk of death being the smallest in those that sleep 7-8 hours a night on average.
After controlling for factors known to increase mortality, such as socio-demographic and lifestyle covariates (marital status, education level, smoking, Body Mass Index, alcohol consumption, physical activity, life satisfaction and history of snoring) the authors found a significant increase in risk of mortality in those who slept less than 7 hours, or a 26% increase in men and 21% in women. They also found increased risk of mortality in those who slept more than 8 hours a night on average or 24% and 17%, respectively.
There are known limitations to self-reports of sleep length, as it may be possible to confuse time in bed with actual sleep time.

Use of Hypnotics or Tranquilizers Increases the Risk of Mortality

Another significant finding in this study was that frequent use of sleep promoting medications increased risk of mortality in both short and long sleepers. Frequent use of hypnotics or tranquilizers increased the risk of mortality by 31% in men and 39% in women. The authors state, however, that from 1975 to 2005 in Finland, the use of hypnotics tripled and the use of tranquilizers doubled.

As the authors point out, there is a complex link between sleep and mortality, where risk is increased in those who sleep less than 7 hours and in those who sleep more than 8 hours a night. In spite of the complexity of this association, it is clear from this study that sleep is a key component to health outcomes.