Both too much and too little sleep can lead to poor health.
An analysis of pooled data from dozens of studies covering more than 3 million people finds that self-reported sleep duration outside of 7–8 hours each night is linked to a higher risk of death and cardiovascular diseases.
The study — which scientists at Keele University in the United Kingdom led and which is now published in the Journal of the American Heart Association — found a "J-shaped" relationship between duration of sleep and deaths. The same relationship was observed with incidents of cardiovascular illness.
The authors say that their findings suggest that sleeping for more than 7–8 hours "may be associated with a moderate degree of harm" compared with sleeping less.
The J-shaped relationship showed that the size of the risk rose in line with greater duration of sleep. Sleeping for 9 hours, for example, carried a 14 percent higher risk of death, while 10-hour sleeps carried a 30 percent higher risk.
The results also showed that poor-quality sleep was linked to a 44 percent higher risk of coronary heart disease.
"Our study," says lead study author Dr. Chun Shing Kwok, a clinical lecturer in cardiology at Keele University, "has an important public health impact in that it shows that excessive sleep is a marker of elevated cardiovascular risk."
Cardiovascular diseases and events
Cardiovascular disease is an umbrella term for disorders of the heart and blood vessels. The heart provides the pressure for pumping the blood through the vessels that carry it to all parts of the body.
Some of the diseases overlap due to a common underlying condition. Atherosclerosis, for example, is an inflammatory condition in which plaque builds up in artery walls and restricts blood flow. This can lead to heart disease, heart attack, and stroke.
Heart failure, in which the heart does not pump enough blood to meet the body's needs, is another type of cardiovascular disease. Abnormal heartbeat, or arrhythmia, and defective heart valves are also classed as cardiovascular diseases.
In the United States, where 610,000 people die of it every year, heart disease is the primary cause of death in men and women.
Also, every year in the U.S., around 735,000 people experience a heart attack. This number includes 210,000 people for whom it is not their first.
In their analysis, the researchers focused on links between sleep, rates of death, and cardiovascular events such as heart attack, stroke, and coronary heart disease.
Sleep duration and quality
In their study paper, the researchers explain that while there appears to be "growing evidence" to support the idea, current guidelines for reducing the risk of cardiovascular diseases "make limited recommendations" about duration and quality of sleep.
The National Sleep Foundation's latest guidelines recommend 7–9 hours of sleep every night for adults aged 26–64, and 7–8 hours for older people.
For their study, the researchers used 7–8 hours as the recommendation against which to compare the various results.
They note that while previous studies had examined the relationship between hours of sleep, deaths, and cardiovascular disease, they had not evaluated the effect of each hour of increase or decrease on the relationship. Also, none had evaluated the effect of sleep quality.
They suggest that their findings are significant because they highlight a problem with longer as opposed to shorter sleep, and that the greater the duration of sleep, the more severe the problem appears to be.
They also note that their study is the first to declare sleep quality as a risk factor for coronary heart disease and suggest that doctors should take into account "nonrestorative sleep (or 'waking up unrefreshed')" when they assess their patients.
Mechanisms are poorly understood
The biology of the relationship between sleep duration, sleep quality, and cardiovascular diseases and mortality is unclear. Those who have studied it conclude that it is complex and involves many factors that interact with each other.
There is some evidence that insufficient sleep raises levels of the energy and appetite hormones leptin and ghrelin and that this can lead to obesity and impaired control of blood sugar. Reduced sleep can also promote inflammation, which some have linked to cardiovascular diseases and cancer.
Scientists also tie low physical activity, depression, unemployment, and low socioeconomic status to longer sleep. These may contribute to, but they could also mask, the link between longer sleep duration and cardiovascular disease and risk of death.
Dr. Kwok remarks that in modern society sleep is subject to many "cultural, social, psychological, behavioral, pathophysiological, and environmental influences."
These influences arise for many different reasons. He lists examples ranging from caring for children and other relatives, to shift-working, mental and physical illnesses, and the "24-hour availability of commodities."
"Our findings have important implications as clinicians should have greater consideration for exploring sleep duration and quality during consultations."
Dr. Chun Shing Kwok