Think it’s all right to get by on just four to five hours of sleep a day? Be warned that medical research does not support that view. By Michael Lim

THERE are people who tell me that they make do with just four to five hours of sleep a day and don’t feel fatigued. Then there are others who sleep more than 10 hours a day and yet continue to feel tired. Many ask: “What duration of sleep is considered adequate sleep?” Is there an optimal number of hours that one should sleep? The answer, apparently, is yes, and sleep does appear to have a significant impact on health.

Cancer

What is consistent in most of the large studies on sleep is that people who sleep the least appear to be significantly more likely to die. A large study, the Nurses Health Study by Harvard University, involving 82,000 nurses, has shown an association between insufficient or erratic sleep and breast cancer, colon cancer, heart disease and diabetes.

While researchers have found that people who work at night are unusually prone to breast and colon cancer, it is still too early to say that working on the night shift causes cancer. All it means is that there is an association. At this point of time, there is no explanation for this association.

Heart disease

However, the relationship between insufficient sleep and an increased risk of heart disease appears stronger. Most of the studies indicate that an increased risk for heart disease starts when people get less than six to seven hours of sleep. In the Nurses Health Study, women who slept five hours or less were 45 per cent more likely to get heart disease compared to those who slept at least seven hours.

In another study by West Virginia University, sleeping less than five hours a day, including naps, more than doubled the risk of being diagnosed with angina, blockage of heart arteries, heart attack or stroke. Those most at risk were adults under the age of 60 who slept five hours or less a night. They increased their risk of developing cardiovascular disease more than threefold compared to people who slept for at least seven hours a day.

Stroke In addition to the increased risk of heart disease, post-menopausal women who slept six hours or less had a 14 per cent greater stroke risk than those who slept seven hours a night. After accounting for the common risk factors for stroke, the increased relative stroke risks compared to the seven-hour-sleep group were 24 per cent for eight hours of sleep and 70 per cent for nine or more hours of sleep. While excessive sleep is associated with increased risk of stroke, the data currently does not imply that excessive sleep causes stroke; the exact mechanism is being studied.

Possible explanations

Possible mechanisms that may explain these risks are that a lack of sleep affects hormonal and metabolic functions, leading to an increase in the appetite-stimulating hormone ghrelin, impaired glucose tolerance, reduced insulin sensitivity and elevated blood pressure, all of which increase the risk of hardening the arteries. These changes increase the regulation of the appetite-related hormone which may contribute to obesity and diabetes.

In a five-year study by the University of Chicago, the percentage of those who slept less than five hours, five to seven hours, and more than seven hours, and who had developed calcification of the heart arteries, was 27 per cent, 11 per cent and 6 per cent respectively. Calcium build-up in the wall of the heart arteries increases the likelihood of plaque build-up and hence also increases the likelihood of heart attacks. So there is strong evidence pointing to an increased risk of heart disease in those with insufficient sleep.

In addition, individuals who had sleep deprivation had higher levels of inflammatory markers: fibrinogen, IL-6 and C-reactive protein. People whose C-reactive protein levels are in the upper third of the population (above 3 milligrams per litre) have roughly double the risk of a heart attack, compared with people with lower C-reactive protein levels, according to the American Heart Association and Centers for Disease Control and Prevention.

Too much sleep

If you think that sleeping more will improve your health, the data will surprise you. The studies generally show that sleeping more than seven hours also increases the risk of heart disease. Those who slept nine hours or longer a day were one-and-a-half times more likely than seven-hour sleepers to develop heart disease.

In addition to the West Virginia University study, there was also data in the Nurses study that pointed towards an association between excessive sleep and heart disease. The mechanisms are uncertain. However, this may partially be because patients with obstructive sleep apnoea (OSA) tend to sleep more and are known to have an increased risk of heart disease. OSA is a condition associated with snoring and reduced oxygen during sleep as a result of partial obstruction of the airway during sleep.

Researchers at Universit√© Laval’s Faculty of Medicine have found that people who sleep too much are also at greater risk of developing type 2 diabetes or impaired glucose tolerance. The risk is two-and-a-half times higher for people who sleep more than eight hours a night. Other studies have also shown that excessive sleep is associated with an increased risk of stroke and death. However, researchers are still uncertain about the mechanisms as to why excessive sleep is associated with stroke and death.

Magic number

The American Academy of Sleep Medicine recommends that most adults get about seven to eight hours of sleep each night. Previous research has shown that people who sleep between seven and eight hours per night live longest. Therefore, it is worthwhile to invest in adequate good quality sleep.

What is consistent in most of the large studies on sleep is that people who sleep the least appear to be significantly more likely to die.