SLEEP and CARDIOVASCULAR RISK
Abstract: Regular habits of sleeping more or less than 7-8 hours each day can produce substantially increased risk of both coronary heart disease and stroke. Those having differing sleeping habits should be aware of their increased risk. If sleeping habits cannot be improved, they should try to compensate via increased attention to other healthful habits.
An interesting finding from one of largest of studies of health is the striking relationship between risk of both coronary heart disease and stroke with usual time of sleeping. This classic study was that of Hammond that followed the death rates of more than a million US men and women. (Arch Environ Health 1969, 19:167). Although this research was done several decades back, the consistency of results for different age and amount of sleep categories was high. Hammond listed risk ratios for each amount of sleep for age levels from 40-49 to 70-79. Similar results for found for individuals of each age level, and no clear pattern of sleeping risk vs. age was observed. Another much more recent publication from the large US Nurses’ study of about 71,600 women confirmed the Hammond findings on 934 events of coronary disease for women. (Ayas,MD Arch Intern Med 2003, 163:205)
The optimum sleep time for both men and women was 7 hours per day for Hammond and 8 hours per day for Ayas. The risk ratios of coronary disease and stroke relative to these optimum times were:
Hammond:
Usual Daily Hours of Sleep <7 7 8 9 10+
Risk of Coronary Heart Disease
Men 1.08 1.00 1.11 1.12 1.41
Women 1.17 1.00 1.27 1.29 1.80
Risk of Stroke
Men 1.02 1.00 1.21 1.39 2.40
Women 1.30 1.00 1.20 1.48 2.25
Ayas: For Women
Usual Daily Hours of Sleep <5 6 7 8 9+
Risk of Coronary heart Disease 1.45 1.18 1.09 1.00 1.38
Considering results from both studies, risks of from 7 to 8 hours of sleep appeared lowest and similar. But an increase to 9 hours moved coronary risks up about 20% to 1.2, and more than 10 hours of sleep suggests at least a 50% increase in coronary risk. The Hammond results suggest that risks of stroke are higher than are risks of coronary disease for differences in sleeping time. A few other studies found generally similar results but that did not add usefully to the findings of the two above large studies.
Life Ahead now uses the following risks for sleep that were derived from the above research
Hours of Sleep 5 or less 6 7 8 9 10 or more
Risk of Coronary Disease 1.45 1.20 1.0 1.0 1.20 1.50
Risk of Stroke 1.60 1.30 1.0 1.0 1.40 2.0
The mechanism via which amounts of sleep affect risks of major disease is not understood. Quite a bit of research relates the effect of sleep apnea on the role of sleep and this may be involved to some extent. There is no clear relationship with sleep and other risk factors such as cholesterol. The increased risks at high amounts of sleep could be partly due to poorer cardiofitness. For example, an average man develops about 950 calories of physical activity during a usual 16 hours awake. Only 13 hours awake because of more sleep would reduce this expected physical activity assuming a similar amount per hour to 780 activity calories per day. This would develop a loss of about 1200 activity calories per week that in turn would reduce cardiofitness by an estimated 4 HEF. This could explain an added risk of about 1.25 times. But this explains only part of the effect of high amounts of sleeping, and does not explain why low amounts of sleep also increase risk.
Thus unusual hours of sleeping indicate some type of a problem that appears unrelated to other known risk factors for these important diseases. Those who have unusual sleep habits should be aware that this increases overall risk of health and the need for increased attention to reducing risks that are achievable from best habits. Life Ahead notes and quantifies the risks for unhealthful sleeping habits.