“Lack of sleep is a 'ticking time bomb',” The Independent reported. The newspaper said that people who regularly sleep less than six hours a night “have a 48 per cent greater chance of developing or dying from heart disease”.
The news is based on research that combined data on almost 475,000 adults, drawn from 15 studies on sleep duration and the risk of strokes and heart attacks. The review found that, compared with a normal 7-8 hours’ sleep a night, shorter or longer sleep was associated with increased risk of these heart problems.
The review has some important limitations. For example, many medical, psychological and lifestyle factors can affect both sleep and cardiovascular health but attempts to account for the influence of these factors varied widely between the studies. It is also unclear whether the participants did not have any cardiovascular disease at the start of the studies, so it should not be assumed that poor sleep was the cause of the cardiovascular problems eventually observed. As the researchers say, the reasons behind any associations between sleep and cardiovascular disease are not fully understood.
Where did the story come from?
The study was carried out by researchers from Warwick Medical School and the University of Naples in Italy. No sources of funding were reported. The study was published in the peer-reviewedEuropean Heart Journal.
The newspapers generally reflected the findings of the research accurately, but did not address the wider issues and limitations of the study.
A systematic review involves searching the global literature to identify all cohort studies relevant to the question of interest. It is the best way of combining all the evidence available to date on how an exposure (in this case sleep duration) relates to an outcome (in this case cardiovascular disease). The process involves the pooling of studies, which will inherently have different designs, methods and assessment outcomes. These differences can potentially lead to limitations in the results of systematic reviews.
It was important that this review studied people who were considered to have developed new cardiovascular disease during the follow-up period. To ensure that participants had truly developed the condition during the follow-up period and not before the study, the studies should have made sure that participants were truly free from disease at the start (baseline). This systematic review did not report whether the individual studies did this.
What did the research involve?
The researchers searched medical literature databases to identify prospective cohort studies published up to June 2009. These studies assessed the duration of sleep at baseline, then followed participants for at least three years to check for any recorded coronary heart disease (CHD), stroke or cardiovascular disease events, or death from these diseases.
Studies were required to have included only adults and to have recorded the number of cardiovascular outcomes that occurred in relation to different ranges of sleep duration. Most studies classed the duration of “normal sleep” as 7-8 hours a night, “short sleep” as less than or equal to 5-6 hours a night and “long sleep” as more than 8-9 hours. In this review, normal sleep was regarded as the reference category, which means that the effects of other sleeping durations were reported in relation to the effect of normal sleep.
After assessing the quality of the gathered studies, the researchers pooled risk figures for the associations between sleep duration and cardiovascular disease development, as well as death from cardiovascular disease.
The study did not give full details of the methods used, although the authors refer to a related 2010 publication that they wrote. This original publication (which searched for studies published up until March 2009) primarily identified studies that had recorded death due to any cause, which was the focus of the researchers’ first review and meta-analysis. It found that, compared with normal sleep, short and long sleep was associated with increased risk of death from any cause. A new search was conducted for this second publication, which specifically focused on deaths or disease attributed to cardiovascular causes.
The current review reported that all studies included had assessed death through death certificates and that non-fatal vascular events (such as strokes and heart attacks) were recorded through disease registers. As these were specific, recorded medical events, we can be sure that they occurred after the original assessment of sleep behaviour and, therefore, after certain sleep patterns.
However, it would be more difficult to reliably examine any association between sleep duration and the development of new cardiovascular disease. The review does not tell us whether the individual studies gave the participants clinical checks to confirm they were free from the condition at the start of the study. This is problematic as, without knowing the details of the numerous individual studies, we cannot rule out that the condition preceded or even influenced the participants’ sleeping behaviours.
What were the basic results?
The review included 15 studies, reporting on 24 cohorts (including some studies also featured in the researchers' 2010 review). These covered 474,684 adults from eight different countries. Four of the studies investigated women only, and the other 11 covered a mixed population. Duration of follow-up varied from 6.9 to 25 years. All studies assessed sleep duration using questionnaires and deaths by looking at death certificates. Non-fatal, new cases of cardiovascular events were recorded through disease registers. The total number of cardiovascular events reported (assumed to include both fatal and non-fatal events) was 16,067 (4,169 cases of CHD, 3,478 strokes, and a further 8,420 cases recorded as any cardiovascular event).
When the researchers analysed their pooled results, they found that short sleep, compared to normal sleep, was reported to be associated with increased risk of developing or dying from CHD (relative risk [RR] 1.48, 95% confidence interval [CI] 1.22 to 1.80), as was long sleep (RR 1.38, 95% CI 1.15 to 1.66). Pooled analysis similarly found that long sleep was associated with increased risk of developing or dying from stroke (RR 1.65, 95% CI 1.45 to 1.87). The increase in stroke risk with short sleep was only just statistically significant (RR 1.15, 95% 1.00 to 1.31). For studies examining total cardiovascular disease, the researchers found that, compared with normal sleep, long sleep was associated with increased risk of developing or dying from any cardiovascular disease (RR 1.41, 95% CI 1.19 to 1.68). There was no association between short sleep and any cardiovascular disease (RR 1.03, 95% CI 0.93 to 1.15).
How did the researchers interpret the results?
The researchers concluded that their review found that shorter-than-normal or longer-than-normal sleep was associated with increased risk of “developing or dying of coronary heart disease and stroke”.
This study found that, compared with 7-8 hours of sleep a night, shorter and longer sleep was associated with increased risk of fatal or non-fatal coronary heart disease or stroke.
There are some important points to consider when interpreting this research:
The review does not specify whether the identified cohort studies excluded existing cardiovascular disease at baseline or looked for new disease development during follow-up. Therefore, it is not clear how reliably it can tell us whether sleep duration is associated with the development of cardiovascular disease.
The participants reported their own sleep duration, which was only measured at one point at the beginning of the study. It cannot easily be assumed that this represents a life-long sleeping pattern for the subject. Also, it is not clear whether all respondents reported sleep in a similar way, for example whether they only considered time in bed or time asleep, including naps as well.
The studies included in the meta-analyses had some variation in their methods. They varied in the time period they assessed (studies commenced between 1970 and 1999), age range of their included population (varying between studies from people aged 31 and over to people aged 69 and over), duration of follow-up (from 6.9 to 25 years) and methods of outcome assessment.
Many factors may affect sleep duration and quality of sleep, including illness, mental health and a person’s life circumstances. The individual studies variably accounted for the participants’ lifestyle, medical and psychological health at the time of assessing sleep, including smoking status, raised blood pressure, diabetes and stress. Such variable lifestyle, medical and psychological health factors could affect the relationship between sleep duration and cardiovascular disease (for example, stress could be the cause of both poor sleep and poor cardiovascular health).
Confirmation that extremes of sleep, both long and short, are associated with poor cardiovascular outcomes is of interest. However, as the researchers say, the “mechanisms that underlie these associations are not fully understood”. As such, the reasons for poor sleep patterns also need consideration, as sleeping for longer or shorter periods may only be a by-product of factors that also affect cardiovascular disease and death.