Sleep and heart attack risk

“Sleeping for less than seven and a half hours a night can raise the risk of heart disease by up to four times” reports the Daily Mail today. It said that in a study of 1,255 people with high blood pressure, those who got under 7.5 hours of sleep and whose blood pressure did not increase at night were 27% more likely to have a heart attack or stroke. Those whose blood pressure also rose at night were at even greater risk and were four times more likely to have an event.

This large study shows a plausible link between sleep duration and cardiovascular risk in people who have high blood pressure. However, it has some limitations and rather than lack of sleep, other factors such as stress and blood pressure may be affecting the apparent increase in risk. In addition, as only a small proportion of the group were at the highest risk (less than 7.5 hours sleep with rising blood pressure at night) these results should be treated with some caution.

Although it is not clear from this study whether people who sleep less than 7.5 hours of sleep can reduce their risk by sleeping for longer, a good night’s sleep is important. People who are concerned about risk of a heart attack should consult their doctor.

Where did the story come from?

Dr Kazuo Eguchi and colleagues from Jichi Medical University and universities in the USA carried out this research. The work was funded by Foundation for the Development of the Community, Tochigi, Japan, Banyu Life Science Foundation International, and the National Heart, Lung, and Blood Institute. The study was published in the peer-reviewed Archives of Internal Medicine.

What kind of scientific study was this?

This prospective cohort study looked at the relationship between sleep patterns and risk of cardiovascular events (including stroke, fatal and non-fatal heart attacks, and sudden death from heart-related causes). Blood pressure usually reduces (dips) during sleep, and it is thought that people whose blood pressure does not drop or indeed rises during sleep may be at greater risk of cardiovascular events. The researchers also looked at whether the relationship was affected by whether an individual’s blood pressure dipped or rose during sleep.

The researchers enrolled 1,268 people from nine medical institutions in Japan, who had all been referred for assessment of their blood pressure between 1990 and 2002. These people were originally recruited as part of two separate studies (the Jichi Medical School study and the Karatsu –Nishiarita study), but were analysed together for the current report. The researchers discounted anyone with kidney problems, liver damage, type 1 or secondary diabetes, ischaemic heart disease, stroke, or other major diseases.

At the beginning of the study, all the participants had ambulatory blood pressure (ABP) monitoring. This involves wearing a monitor that measures and records blood pressure every 30 minutes for 24 hours. The participants recorded the time they went to sleep and got up in a diary. The researchers used the ABP data and sleep diaries to identify the participants’ whose blood pressure fell less than 10% on sleeping (called non-dippers), and those whose blood pressure did not fall at all (called risers). People who reported that the ABP monitoring disturbed their sleep were excluded from the study.

In all, 1,255 participants provided complete data and were included in the analyses. These participants’ average age was 70.4 years, and 94% had high blood pressure. The participants were followed for up to 5.7 years (the Jichi Medical School study) or 9.7 years (the Karatsu –Nishiarita study). Their medical records were reviewed annually to identify anyone who had experienced a stroke, fatal and non-fatal heart attack, and sudden death from heart-related causes. Participants who did not visit the clinic were interviewed on the telephone. Diagnoses were made by their doctor, and confirmed by independent neurologists and cardiologists.

The researchers then looked at whether experiencing a cardiovascular event had any relation to sleeping pattern, and whether blood pressure pattern during sleeping affected this. The analyses were adjusted for factors that might affect sleep duration or risk of cardiovascular events, such as age, gender, body mass index (BMI), smoking, level of cholesterol and other fats in the blood, and average systolic blood pressure.

What were the results of the study?

Half of the participants in the study slept less than 8.5 hours, and a quarter slept under 7.5 hours a night. People who slept more tended to be older, have lower BMIs, and pulse rates, and were less likely to have diabetes. The participants were followed for an average of 50 months, and during this time there were 99 cardiovascular events.

Overall, people who slept less than 7.5 hours a night were about 60% more likely to experience a cardiovascular event than those who slept for longer. The risk of having an event in people who slept less than 7.5 hours a night was 2.4% per year on average, compared with 1.8% in people who slept longer.

About 8% of participants did not experience a fall in blood pressure as they slept. People who had this characteristic and also slept less than 7.5 hours a night were at the greatest risk of cardiovascular events. These people had about four times the risk of having an event compared to people who slept for 7.5 hours of more and whose blood pressure did fall while asleep.

What interpretations did the researchers draw from these results?

The researchers concluded that shorter periods of sleep are associated with an increased risk of cardiovascular events independent of other factors. They suggest that doctors should ask patients with hypertension about their sleep duration in order to help assess their risk of cardiovascular events.

What does the NHS Knowledge Service make of this study?

This large study has shown an association between sleep duration and subsequent cardiovascular risk among Japanese patients with hypertension. There are a few points to consider when interpreting its results:

• It is unclear if there was just one measurement of the participants’ ambulatory blood pressure and sleep duration. If only a single measurement was taken, it might not be representative of the patient’s usual blood pressure or sleep patterns.
• The results are from a Japanese population, and so the results may not be applicable to other populations. In addition, the study population mostly had high blood pressure, which already puts them at higher risk of cardiovascular events. Therefore, these results may not apply to people who do not have high blood pressure.
• As with all studies of this type, there may be differences between the groups other than what is being examined (in this case sleep duration) that cause associations to be observed. The researchers attempted to reduce the risk of this happening by taking into account some of the known risk factors for cardiovascular events. Although this increases the confidence that can be had in the results,  there are still some other factors which could be responsible for the association seen. For example, people who sleep less may do so because they work longer hours or are more stressed, and these factors could be contributing to the association. In addition, it is unclear how well the participants’ high blood pressure was controlled over the follow up period, and if this was the same in those who slept differing amounts. Poorly controlled high blood pressure would increase cardiovascular risk.
• Only 20 patients had both the “riser” blood pressure pattern and slept less than 7.5 hours a night. The large increase in risk of cardiovascular events found in this group should be interpreted with caution as it is based on such a small number of people.
• The study did not investigate whether increasing the duration of sleep would reduce people’s risk of cardiovascular events, so no conclusions can be drawn about this.

In general, this association seems a plausible one, but it may be the causes of lack of sleep, rather than the lack of sleep itself that is causing the increase in risk.

Sir Muir Gray adds...

Eight hours a night sounds good, but not as important as stopping smoking and other premier league risk factors.