Sleep and heart risk link is uncertain

reposted from: http://www.nhs.uk/news/2011/02February/Pages/sleep-patterns-predict-heart-attack-risk.aspx

crabsallover highlights, key points, comments / links.

“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-reviewed European Heart Journal.

The newspapers generally reflected the findings of the research accurately, but did not address the wider issues and limitations of the study.

What kind of research was this?

This systematic review and meta-analysis combined observational studies that had assessed the relationship between duration of sleep and later development of coronary heart disease (CHD) or stroke, as well as the risk of death from these diseases.

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”.

Conclusion

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.

Links to the headlines

Lack of sleep is a 'ticking time bomb'. The Independent, Februaury 9 2011

Why late to bed, early to rise is a recipe for a heart attack. Daily Mail, Februaury 9 2011

Warwick scientists say lack of sleep 'bad for health'. BBC News, Februaury 9 2011

Links to the science

Cappuccio FP, Cooper D, D'Elia L et al. Sleep duration predicts cardiovascular outcomes: a systematic review and meta-analysis of prospective studies. European Heart Journal, February 7 2011 (first published online)

Sleep hours 'linked to death'

reposted from: NHS Choices

NHS Choices Mon Oct 04 2010 17:50:00 GMT+0100 (GMT Daylight Time) Share

“Women who get between five and six-and-a-half hours sleep a night could live longer,” reports the Daily Express. The news is based on the results of a long-running study of 459 women aged 50 to 81.

This research initially assessed the women’s sleep patterns over one week, using wrist-mounted activity monitor worn at night. Researchers then followed the women for up to 14 years to see how their sleep patterns might have influenced their chances of survival. The scientists found that those women who had slept longer or shorter hours were more likely to have died compared to women who had slept a moderate length. However, as with all studies of this type, it is difficult to say for certain whether sleep directly caused differences in survival rates. Also, the study’s findings may not apply to men or to people under 50 years old.

Although this and other studies seem to suggest that sleeping too much or too little is associated with an increased risk of death, it is not possible to say why this might be, nor whether changing your sleep patterns could influence your longevity.

Where did the story come from?

The study was carried out by researchers from the University of California and the Jackson Hole Center for Preventive Medicine in Wyoming. It was funded by the US National Institutes of Health, and it was published in the peer-reviewed journal Sleep Medicine.

The Daily Express and Daily Mail reported this study. Both summarised the results accurately.

What kind of research was this?

This was an analysis of data from a long-running cohort study called the Women’s Health Initiative (WHI). It aimed to assess the relationship between length of sleep and the risk of death during the follow-up period.

Several previous studies have suggested that sleeping for a long time (e.g. more than 7.5 hours) or for a short time (e.g. less than 6.5 hours) are associated with an increased risk of death compared with an intermediate amount of sleep. However, these studies mainly used subjective reports of sleep, where a person reports their own sleep patterns. The current study wanted to assess whether objectively measured sleep length was also related to risk of death. A previous study looking at objectively measured sleep did not find any evidence of a link.

This type of study design is appropriate for addressing the question asked, as it would not be feasible to use a study design that would randomly assign people to differing amounts of sleep over a long period of time.

As with all observational studies, the main danger is that factors other than the one being studied may be influencing the results. For example, if people who slept for only short periods or long periods also had unhealthier lifestyles, this could be influencing their risk of death rather than their sleep patterns. It is important that this type of study takes this possibility into account.

What did the research involve?

Between October 1995 and June 1999, the researchers asked 451 women taking part in the WHI study to attach to their wrists an activity monitor called an ‘actigraph’. They used the data from these actigraphs to identify the periods when the women were asleep. They then followed the women for up to 14 years to see who died and, if so, when. The researchers then carried out analyses to see if there was a relationship between death and sleep duration.

The researchers selected a sample of women to take part. It included a high proportion of older women, and of women reporting six hours sleep or less, or eight hours sleep or more. This was intended to increase the likelihood that they could detect an effect of sleep length on risk of death if there was one. The women’s average age at the start of the study was 67.6 years (range 50 to 81 years).

The women filled in sleep questionnaires and had psychiatric interviews. The researchers also had access to the questionnaires the women had filled at the start of the original WHI study about their health and lifestyles. All participants wore the actigraph on their wrist for seven days and nights. They also completed a sleep diary and estimated their sleep duration over these seven days. Most participants had urine samples collected over 24 hours, and they wore an oxygen saturation monitor for three nights to identify any sleep apnoea.

The women were mailed annual questionnaires and contacted by telephone until 2005. Any deaths were indentified in this way. In 2009, any additional deaths were identified using the Social Security Death Index. The final set of analyses used follow-up data available for 444 women, 98% of the study population.

The analyses compared the survival of women who slept for different lengths of time. For example, they compared women with 300–390 min of sleep against women who had less than 300 min or longer than 390 min. The researchers took into account factors that could affect results (potential confounders). This included age, history of high blood pressure, diabetes, heart attack, cancer and major depression.

What were the basic results?

Among the 444 women with follow-up data, 86 women had died over an average of 10.5 years of follow-up. According to actigraph readings, the women slept an average of about six hours, which was shorter than the average length of sleep according to the women’s estimates in their sleep diaries, which was 6.88 hours.

The researchers estimated that women who slept less than five hours a night had a 61% chance of survival at the end of follow-up, compared with 78% among those who slept longer than 6.5 hours, and 90% for those who slept between five and 6.5 hours a night. When taking into account all potential confounders, there was a significant link between sleep duration and risk of death. However, the link was only just statistically significant when looking at sleep duration as a continuous outcome, i.e. the relationship seen across all sleep durations.

Medical factors such as high blood pressure, diabetes, having previous heart attack or cancer, or having major depression at the start of the study appeared to have a greater influence on risk of death.

How did the researchers interpret the results?

The researchers concluded that their study confirmed the “U-shaped” relationship between sleep duration and risk of death. Short and long sleep durations were both associated with an increased chance of death compared with intermediate durations. They say that more research is needed to identify approaches that might prevent this increased risk of death.

Conclusion

This study’s strengths include the use of an objective measure of sleep, and the long period of follow-up. It supports the findings of other studies with subjective sleep measures, which suggest that very long or very short sleep duration is linked to an increased risk of an earlier death. There are some important points to note:

• The data is for women aged 50-and-over only, and may not apply to men or to younger age groups.
• The actigraph measures movement, so it would not be able to tell the difference between someone lying still while awake and someone lying still while asleep. This may have caused some inaccuracy in the estimation of how long people were asleep, although possibly less than if a subjective measure of sleep was used. The researchers acknowledge that using actigraphs may over- or under-estimate sleep compared to the gold standard way of measuring sleep (called polysomnography).
• The study only measured sleep using the actigraph for one week at the start of the study. Women’s sleep in this period may not have been representative of their sleep patterns over a lifetime.
• It is not possible to say whether duration of sleep is itself affecting the risk of death, or whether another unknown factor is behind both the different sleep patterns and risk of death. Although researchers took into account a number of factors that could affect risk of death, these or other unmeasured factors could still be having an effect.
• Although studies subjectively measuring sleep duration have found a link, a study objectively measuring sleep using the gold standard objective (called polysomnography) did not find a link between short sleep and increased risk of death.

How long we sleep may be affected by many factors, including our internal body clock, our jobs and families, lifestyles, the environment in which we sleep, and stress levels.

Although this and other studies have suggested that sleep duration is linked to risk of death, it is not yet possible to say whether you can improve your longevity by simply modifying how long you sleep, without changing any other factors.

Links To The Headlines

Sleep less...and live longer. Daily Express, October 2 2010

Only manage five hours of sleep? It's the ideal amount for women who want to live longer. Daily Mail, October 2 2010

Links To Science

Kripke DF, Langer RD, Elliott JA et al. Mortality related to actigraphic long and short sleep. Sleep Medicine 2010

Avoiding cancer: Top tips from the experts

World class advice on how to sidestep cancer

Cancer special: Expert tips

• 22 October 2008
• From New Scientist Print Edition.

Drink green tea

"Breast cancer is the top cancer among women in Singapore. I try to reduce my risk by drinking more green tea and eating more soy-based products. I also steam my tofu with fish since omega-3 fatty acids have been shown to reduce breast-cancer risk. Finally, I try to catch up on sleep because this may reduce my chance of breast cancer too, if our recent findings are true"

Woon-Puay Koh, cancer epidemiologist, National University of Singapore

Take aspirin

"I take a baby aspirin each morning. I eat a largely vegetarian diet, and I am a fanatic, but not totally successful, in trying to keep my weight down. They say that obesity is the second most important risk factor for cancer deaths after tobacco use. I feel increasingly, in the words of the old hippies, that we are what we eat. It's still a throw of the dice, but at least we can tilt the playing field in our favour"

Robert Weinberg, Whitehead Institute for Biomedical Research, Cambridge, Massachusetts, discoverer of the first oncogene

Get screened

"I take low-dose aspirin - it does reduce your risk of colon cancer by about 40 or 50 per cent.

I also abide by all of the screening tests. Unfortunately, a lot of the population can't afford that kind of intervention, so we've got to figure out ways to make some of these tests less costly"

Ray DuBois, M. D. Anderson Cancer Center, Houston, Texas, and president of the American Association for Cancer Research

Eat your greens

"I grew up in Nigeria and that upbringing instilled behaviours that still make good healthy sense. We ate lots of fruits and vegetables and not much red meat. We got exercise, not always by choice. I now have modern conveniences but I still exercise, by choice. And despite my African heritage, I still try to reduce sun exposure"

Funmi Olopade, director of the University of Chicago's Cancer Risk Clinic

Keep your weight down

"I have never smoked, I'm not overweight, I avoid getting sunburnt and I don't drink much alcohol. I also cycle or walk most days. I have been a vegan for over 30 years because I don't like slaughterhouses. But whether this has any effect on cancer risk remains to be seen"

Tim Key of Cancer Research UK's Cancer Epidemiology Unit, University of Oxford

Eat a balanced diet

"There are many nutrients that are thought to be potentially protective for cancer, including vitamin D, selenium, carotenoids, other antioxidants, and specific fatty acids. However, I don't take any dietary supplements because I do not think there is strong evidence that supplements have benefits for cancer beyond the nutrients that would be in a varied and balanced diet"

Kay-Tee Khaw, clinical gerontologist, University of Cambridge

Choose your parents carefully

"I don't smoke, and that's the very best thing anyone can do to avoid cancer. I'm a pale-skinned Scotsman, so I avoid the sun where possible and wear sunblock to avoid melanoma. Another tip would be to choose your parents carefully, to check for inheritable cancers, but that's a bit tricky"

Iain McNeish, medical oncologist, Barts and the London School of Medicine

Think positive

"My advice? Well, don't smoke, eat good green stuff, make very good friends with doctors so that they can snip off the odd mole and do the odd colonoscopy. Also, climb mountains and be very happy and positive the whole time"

David Lane, University of Dundee, UK, who showed that defects in the p53 gene cause a range of cancers

Read more in Cancer special: Old killer, new hope

From issue 2679 of New Scientist magazine, 22 October 2008, page 37