The need for sleep
Just what is the optimal amount of sleep and how does too little or too much affect us? Lynnette Hoffman finds out.
HERE’S a not-so revolutionary statement: People feel better when they get enough sleep. Maybe it’s precisely that simplicity that leaves sleep neglected from most public health campaigns, which tend to target diet, exercise, smoking and drinking more aggressively.
That’s despite the slew of research linking adequate sleep to known risk factors of cardiovascular disease (CVD), including obesity, hypertension and depression.
However, now there’s new evidence that getting too much or too little sleep could be a risk factor for CVD in its own right.
Two researchers at West Virginia University (WVU) looked at the self-reported data of 30,397 adults over 18 who had participated in the US National Health Interview Survey 2005 to see how strong a correlation sleep duration had with CVD.1
Sleep duration was found to be independently associated with CVD after adjusting for age, sex, race-ethnicity, smoking, alcohol intake, BMI, physical activity, diabetes, high blood pressure and depression.
Participants who averaged seven hours of sleep each night made up the lowest proportion of those with heart disease.
“Any change by one hour or more, either higher or lower, was associated with increased risk of heart disease,” explains study author Associate Professor Anoop Shankar an epidemiologist at WVU.
“The strongest association was with people who slept for five hours or less [within a 24-hour period].
Their risk was almost two times higher. People who slept more than nine hours had 1.57 times higher risk.”
What may be surprising is that eight hours of sleep per night – often recommended as a healthy amount – was associated with “a very modest but statistically significant” increase in risk, which may not have been detectable in a study with a smaller sample size, Professor Shankar says.
“Seven hours of sleep seems to be ideal, and any deviation from that in either direction shows increased risk. This suggests a dose-response relationship,” he says.
“That suggests there really is a biological mechanism underlying this observed association – but it’s important not to blow things out of proportion. It’s really for those getting less than five or more than nine hours of sleep that there is a strong magnitude of association.”
Short sleep a marker
The researchers did separate analysis excluding confounding factors, such as excluding those with diabetes, hypertension and depression. They found the data remained consistent for all groups.
Professor Shankar theorises that shorter sleep duration could be a marker of undiagnosed sleep apnoea, which is associated with heart disease, and longer sleep duration may also be linked with underlying conditions that have not been picked up, such as hypothyroidism.
Or sleeping too long may also be another manifestation of sleep apnoea, which reduces quality of sleep – meaning some people may stay in bed much longer to try to make up for the fact that they are waking up multiple times in the night.
This was certainly not the first study drawing attention to the more direct role sleep may be playing as a risk factor for CVD. Another high-profile study published in the Journal of the American Medical Association in 2008 found that short sleep duration was linked with increased calcification of the coronary arteries.2
That long-term observational study did not rely on self reported sleep data – the 495 healthy participants, who ranged in age from 35 to 47, wore a special wrist device that measured movement every 30 seconds.
Participants slept with the device for six nights at the outset of the study, and then again a year later, and were followed up at years 15 and 20.
Potential confounders (age, sex, race, education, apnoea risk, smoking status) and mediators (lipids, blood pressure, BMI, diabetes, inflammatory markers, alcohol consumption, depression, hostility and self-reported medical conditions) were measured at both baseline and follow-up.
Ultimately, the differences were marked. Twenty-seven per cent of those who averaged less than five hours of sleep each night showed plaque in their coronary arteries, compared with 11 per cent of those who slept 5-7 hours and only 6% of those who slept more than seven hours.
Interestingly, study author Professor Diane Lauderdale, from the University of Chicago, says that none of the participants actually slept more than nine hours.
This could be due to a number of reasons, she said, including the nature of a 20-year cohort study, which could result in some patients ‘self-selecting’ themselves out.
Professor Lauderdale says researchers have not yet been able to identify the mechanism by which too little sleep could lead to calcification of arteries – and she cautions against putting too much weight on a single observational study.
Both she and Professor Shankar say research directly linking sleep and CVD is in early days.
Does it matter whether the sleep one gets is irregular, accumulated in chunks or different times of day the way a shift worker or a new parent might be more inclined to get it? Can you reverse your risks by changing your habits?
No-one knows at this stage, because the studies have not yet measured those factors; no intervention studies have been performed, and the studies that do exist are observational, each with its own limitations.
As for whether poor sleep is an independent risk factor for CVD, rather than a side-effect of some other underlying condition, Professor Lauderdale says that too remains to be seen.
“We don’t know the answer to that, and I don’t think the studies have confirmed that yet – although sleep researchers like to say it is [an independent risk factor].”
Still the evidence is accumulating, and in some cases it is quite compelling. For example, Professor Lauderdale says there’s a strong link between sleep fragmentation, sleep duration and BMI – and probably the most compelling evidence has to do with sleep disorder and high blood pressure.
“There is much stronger evidence that sleep apnoea causes hypertension and that treating sleep apnoea reduces hypertension,” she states.
Professor Shankar says while there is much more to learn, there’s enough evidence to warrant talking to patients about their sleep habits the way you would other lifestyle factors.
“Along with questions such as ‘How many cigarettes do you regularly smoke?’, and ‘How many drinks do you have?’, primary care physicians should also be asking, ‘How long do you sleep on average?’.
“It’s a marker of underlying problems too,” Professor Shankar says.
“We hope these findings encourage primary care physicians to evaluate sleep patterns of patients as a screening factor for heart disease risk, and we would like to see public health initiatives focused on improving sleep to eventually reduce the burden of heart disease.”