How do you define “sufficient sleep”? Does this length vary for different individuals?
We used 6 hours or less per 24 hours as the definition of insufficient sleep, with 7 hours or more as sufficient sleep. In general, this is considered the amount of sleep that is sufficient for adults, although there can be differences between persons with respect to the amount of sleep needed.
We used the same definition for all respondents. In earlier analyses in a subgroup of respondents in which we also asked about rising rested in the morning, we found that the increased risk for cardiovascular diseases is especially present in those who sleep insufficiently AND indicate they do not rise rested in the morning.
The risk in those with 6 hours of sleep or less who indicate they rise rested has a smaller (not statistically significant) increased risk. However, this was a smaller subgroup, so in the present analysis where we combine sleep duration with the other healthy life styles; we are not able to include also sleep quality (rising rested yes/no).
How was this length determined?
Sleep duration is based on self-report by the respondent. The question that was asked was ‘how many hours do you usually sleep per ‘etmaal’ (Dutch word for 24 hr period). There were 5 answering categories: 5 hours or less; 6; 7; 8; 9 hours or more.
Please can you outline your recent research on the health benefits of sufficient sleep?
Between 1993 and 1997 we examined over 20,000 men and women aged 20-65 years in the Netherlands. It was a broad survey on lifestyle factors, metabolic risk factors and chronic diseases.
We measured metabolic risk factors (such as serum cholesterol, blood pressure and BMI), and asked about lifestyle habits (smoking, physical activity, alcohol consumption), health and educational level using an extensive questionnaire.
A separate 178 item food frequency questionnaire was filled out by the respondents, to assess their usual diet.
Sleep was assessed as described above, with a fairly simple question. For the ‘traditional’ life style factors that are known to be related to cardiovascular diseases, we dichotomized into ‘healthy’ versus ‘unhealthy’ behaviour and counted the number of healthy life style factors for each respondent (not smoking, being physically active, eating a healthy diet based on the Mediterranean Diet Score (MDS) and moderate alcohol consumption).
Subsequently, we assessed whether in addition to already having a healthy lifestyle, having sufficient sleep contributed to further lowering of cardiovascular risk. Because we had such extensive information on life style, we were able to study the impact of sleep on top of other, well-known life style factors.
What were the results of your study?
We found that indeed, having sufficient sleep further lowers cardiovascular risk, beyond the benefits obtained from the ‘traditional’ healthy lifestyle factors.
During 10–14 years of follow up, 607 composite CVD events (fatal CVD, nonfatal myocardial infarction and stroke) occurred, of which 129 were fatal.
Those with the four traditional healthy lifestyle factors had a 57% lower risk of composite CVD (HR 0.43, 95% CI 0.31–0.59) and a 67% lower risk of fatal CVD (HR 0.33, 95% CI 0.16–0.68) compared with those with none or one healthy lifestyle factor.
Sleeping sufficiently in addition to the four traditional lifestyle factors resulted in a 65% lower risk of composite CVD (HR 0.35, 95% CI 0.23–0.52), and an 83% lower risk of fatal CVD (HR 0.17, 95% CI 0.07–0.43).
How do you think the benefits of sleep can be explained?
During sleep a number of restorative processes take place in the body. Short sleep duration has been associated with a higher incidence of overweight, obesity and hypertension and with higher levels of blood pressure, total cholesterol, hemoglobin A (1c) which is a measure of glucose metabolism, and triglycerides.
Mechanisms include activation of low-grade inflammation, and changes in circulating levels of leptin and ghrelin that, in turn, increase appetite and caloric intake, reduce energy expenditure, and facilitate the development of obesity and impaired glycemic control.
Did your study account for the quality of sleep in addition to the quantity?
Sleep quality is also of importance, but for this analyses we did not have information on sleep quality for a large enough sample of our population, to take also quality into account.
However, as described in my answer to the first question, we did find an impact of quality in subsample of our population for which we had the answer to the question ‘do you usually rise rested in the morning ?’
Is further research needed to confirm your results?
In our analyses we looked at the impact of sufficient sleep on top of having four healthy lifestyles. This has not been done before, therefore it is important to confirm results in other studies.
There are two Asian studies that looked at sleep in combination with other healthy lifestyles, but those studies counted sufficient sleep as one of the lifestyle factors (so in those studies, the category of people with 2 healthy lifestyles include e.g. people who do not smoke & eat healthy but do not have sufficient sleep, and also people who do not smoke & sleep sufficiently).
Also, more detailed research is needed on the effect of sleep quality. It is also interesting to know more on the effect of napping: is it the same to sleep 8 hours at night as sleeping 6 hours at night and sleep 2 hours during day time?
What impact do you think your study will have?
I hope that awareness of the importance of sufficient sleep will increase, both in the general public and in public health professionals. It is an additional way to lower your cardiovascular risk.
Some people may suffer from insomnia, than it is of course not easy to simply sleep more hours.
However, also ‘lack of discipline’ can be the reason for not getting enough sleep: staying up to long (in front of the TV or computer). When you stay up to long, and have to go to work in the morning, you don’t have the opportunity to compensate. In that case, awareness of the health benefits of sufficient sleep may make people go to bed in time.
What are your plans for further research?
We are planning to also look at the impact of insufficient sleep on cognitive function. We have also measured cognitive functioning in those over 45 years of age, and we are interested in the effects of sleep on cognitive decline.
Also, we regularly re-examine a smaller sample of our study population, and in more recent examinations we have asked more extensive questions on aspects (quality) of sleep. We will look into that in the near future.
Where can readers find more information?
Our results were published in the European Journal of Preventive Cardiology, a journal of the European Society of Cardiology:
Hoevenaar-Blom MP; Spijkerman AMW, Kromhout D, Verschuren WMM. Sufficient sleep duration contributes to lower cardiovascular disease risk in addition to four classical lifestyle factors: The MORGEN Study. Eur Journal of Preventive Cardiology. Epub ehead of print DOI: 10.1177/2047487313493057
Hoevenaar-Blom MP, Spijkerman AM, Kromhout D, van den Berg JF, Verschuren WM. Sleep Duration and Sleep Quality in Relation to 12-Year Cardiovascular Disease Incidence: The MORGEN Study. Sleep 2011;34:1487-92.
About Dr Monique Verschuren
Monique Verschuren, PhD, is head of the Department of Chronic Diseases Determinants, Centre for Nutrition, Prevention and Health Services of the National Institute for Public Health and the Environment (RIVM), Bilthoven, the Netherlands.
She studied Human Nutrition with a specialization in epidemiology at Wageningen University, the Netherlands, where she also completed her PhD thesis ‘Serum cholesterol – a public health perspective’.
Since 1989 she works at the RIVM, working on the epidemiology of chronic diseases. She is the principal investigator of a number of population based cohort studies, such as the MORGEN-project, on which the above described research is based, and the ongoing Doetinchem Cohort Study.
She is a member of the Steering Committee of the EPIC Study (European Prospective Investigation into Cancer and Nutrition). She has authored about 175 international peer-reviewed papers.
Her main focus of research is on lifestyle factors, diet and metabolic risk factors in relation to cardiovascular diseases and cognitive decline.
She has been a member of the ‘Fifth Joint Task Force of the European Society of Cardiology and Other Societies on Cardiovascular Disease Prevention in Clinical Practice’ that prepared the latest (2012) European Guidelines for Cardiovascular Disease Prevention in Clinical Practice, where she was coordinator of the chapter on lifestyle factors.
Links to website of my institute: www.rivm.nl