Tuesday August 30 2011
The men in the study were 75 years old on average
“Poor sleep makes older men 80% more likely to develop high blood pressure,” reported the Daily Mirror. It said that a three-year study found that “men who spent less than 4% of their time in a deep slumber – known as slow-wave sleep – had more heart problems than other people”.
This study was of men over 65 years of age without high blood pressure. Researchers measured their sleep pattern over one night and looked at their risk of having high blood pressure up to three years later. The less time the men spent in a stage called slow-wave sleep (deep sleep), the greater their risk of developing high blood pressure. The total length of sleep, or being aroused from sleep during the night, was not independently associated with the increased risk.
This was a well-conducted study, but it was limited in that only one recording of the men’s sleep was made. There is also the possibility that some men were incorrectly categorised as having high blood pressure, as this was based on the men reporting their blood pressure status themselves, the use of hypertensive medications or a one-off blood pressure of above 140/90mmHg. This study also did not look at differences between men and women’s sleeping patterns, and investigated sleep in older men only.
These findings are worthy of further study, but alone do not prove conclusively whether, or how, sleep patterns affect blood pressure risk.
Where did the story come from?
The study was carried out by researchers from the University of California, San Diego, and Harvard Medical School. Funding was provided by the US National Institute of Arthritis and Musculoskeletal and Skin Diseases, the National Institute on Aging, the National Center for Research Resources and the National Institutes of Health Roadmap for Medical Research. The research was published in the peer-reviewed journal Hypertension.
The BBC News and the Daily Mirror accurately reported that this study looked at the effects of sleep in older men only and that a particular phase of sleep was associated with an increased risk of high blood pressure. The Daily Mail focused on potential differences between men and women’s sleep patterns and how this could affect the risk of developing high blood pressure and heart disease. However, the current study recruited a male-only cohort and did not look at heart disease outcomes.
What kind of research was this?
This cohort study looked at whether there is an association between sleep quality in elderly men and the risk of developing high blood pressure.
The researchers say that it is thought that sleep disorders and sleep disordered breathing may affect the hormone system and nervous system, thereby potentially contributing to high blood pressure. They also say that epidemiological studies of self-reported sleep have found that sleep deprivation or short sleep duration are associated with high blood pressure. To date, however, there has not been a comprehensive evaluation of sleep characteristics and the onset of high blood pressure in an elderly cohort. The aim of this study was to see whether sleep quality could predict new cases of high blood pressure in elderly men living in the community.
What did the research involve?
The present investigation was an ancillary study of the Outcomes of Sleep Disorders in Older Men Study (also referred to as the Osteoporotic Fractures in Men [MrOS] Sleep Study). The MrOS study recruited a cohort of 5,994 men aged over 65 years between 2003 and 2005. The specific aim of the MrOS study is not reported in this paper. The present study included 784 men with an average age of 75. When they enrolled, the men had had their brain waves recorded by polysomnography over one night while they slept in their home. The men had no history of high blood pressure, were not taking blood pressure medications and did not have an elevated systolic blood pressure (SBP) of over 140mmHg or an elevated diastolic blood pressure of over 90mmHg. The men attended a follow-up visit between 2007 and 2009, where they were assessed for high blood pressure.
High blood pressure was defined as a self-report of high blood pressure, use of high blood pressure medications or a measurement of SBP of over 140mmHg or diastolic blood pressure over 90mmHg (said to be taken by two seated blood pressure measures). The researchers defined an extra category of “pre-high blood pressure” if the person had an SBP of 120-140mmHg or diastolic blood pressure of 80-90mmHg.
The participants were given questionnaires, which asked them about demographic information, lifestyle factors such as physical activity, depression, alcohol and smoking. The medications they were taking were also recorded, as was their height, weight and BMI, as well as their waist, hip and neck circumferences.
What were the basic results?
The researchers found that 54% of the men had sleep disordered breathing at the beginning of the study. They defined this according to the respiratory distress index (number of episodes of minimal or absent breathing recorded per hour of sleep), or by the proportion of sleep time that the person was recorded as having a low blood oxygen level. The average length of time spent asleep was 6.1 hours. On average, the men spent 20.2% of their sleep time in REM sleep and 8.5% in slow-wave sleep, which is considered to be “restorative sleep” and is the sleep stage associated with the highest arousal threshold (i.e. the deepest sleep).
Over the average 3.4-year follow-up interval, 243 men developed high blood pressure and 70% were taking one or more blood pressure medications. There was an association between the percentage of time spent in slow-wave sleep and the two sleep stages that preceded it (called N1 and N2) and new cases of high blood pressure. People who subsequently developed high blood pressure had spent less time in slow-wave sleep (9.8% versus 11.2%, p=0.002) and more time in non-REM N1 and N2 sleep.
The researchers compared the risk of developing high blood pressure in the 25% of men who had spent the lowest time in slow-wave sleep with the risk in the 25% of men who had spent the highest time in slow-wave sleep. Those men who had the lowest duration of slow-wave sleep were 81% more likely to develop high blood pressure in the follow-up period compared with men who spent the most time in this phase of sleep (odds ratio 1.81, 95% CI 1.18 to 2.80). The researchers adjusted these results for age and BMI. The association remained significant, even after factors that might affect it were taken into account, including location, race, history of cardiovascular disease, arousal instances during sleep, sleep time, smoking history and alcohol use.
How did the researchers interpret the results?
The researchers said that older men who spend less sleep time in slow-wave sleep have an increased risk of developing high blood pressure. In contrast, measurements of breathing disturbances, level of hypoxemia (low oxygen content of the blood), sleep duration and arousal index were not associated with an increased risk of high blood pressure.
The researchers said that further studies are necessary to confirm these observations, understand what the mechanism is and to determine if it is possible to improve high blood pressure by modifying the time spent in slow-wave sleep.
This prospective cohort study showed an association between a decreased amount of time spent in slow-wave sleep and an increased risk of developing high blood pressure in elderly men.
One strength of this study was that it used “polysomnography” to measure sleep characteristics. This enabled the researchers to observe that certain phases of sleep seemed to be particularly associated with a higher risk of high blood pressure rather than disrupted sleep in general. However, only one measurement was taken, which may not be representative of an average night’s sleep. As the researchers note, another problem with monitoring sleep in this way is that it may prevent the participant from normal sleep as it involves attaching leads and electrodes to the scalp.
There is also the possibility that some men were wrongly categorised as having high blood pressure, as this diagnosis was obtained through self-report, use of hypertensive medications or a blood pressure of above 140/90mmHg. Though the researchers say that two seated blood pressure measures were taken, they do not say whether these measures were taken on two separate occasions, according to convention. A single blood pressure measurement may not discriminate between people who have high blood pressure at all times and those who have a temporary increase due to the effect of having their blood pressure taken (this is called white coat hypertension).
The researchers also highlight that they did not address residual confounding associated with other medications (besides those used for high blood pressure), activity and comorbid conditions. This study also did not look at differences between men and women’s sleeping patterns, and investigated sleep in older men only.
Overall, this was a well-conducted study that warrants further follow-up to confirm the findings and to understand how sleep may affect blood pressure. Alone, this study does not prove conclusively whether or how sleep patterns affect blood pressure risk.