MEDICAL MATTERS:Restlessness can double cardiovascular risks, writes MUIRIS HOUSTON
ARE YOU a good sleeper? Do you get a regular seven to eight hours’ sleep a night, as recommended by sleep experts? The answer to this question is more than just about feeling well rested and alert during the day – it may actually affect your risk of developing heart disease.
And before those of you who are long sleepers begin to gloat, the latest research points to a link with cardiovascular disease for those at both extremes of the sleep duration spectrum. Researchers from West Virginia University School of Medicine looked at data from more than 30,000 adults and found that 8 per cent of these reported sleeping less than five hours a day. They then calculated the risk of heart disease for the short sleepers was more than twice that of the participants who slept seven hours per day.
In addition, the 9 per cent of those studied who reported sleeping more than nine hours a day had a 1.5 times greater risk of cardiovascular disease than the seven hours a day group. My first reaction when reading the title of the research was that the link was through depression: people with depression tend to be affected by sleep disturbance, and depression is now an acknowledged risk factor for heart disease. However, the authors, writing in the journal Sleep, took account of possible confounding factors such as smoking, diabetes and depression.
A word of caution about the research – it is based on people’s recall of their sleep pattern rather than an objective measurement and it is also what we call retrospective, cross-sectional research, based on a past history of cardiac disease. The most that can be said is it suggests an association between extreme sleep patterns and heart disease; it does not suggest a cause and effect relationship. Still, a possible link does make sense when our knowledge of circadian rhythms is taken into account.
The level at which your body functions oscillates depending on the time of day. This 24-hour cycle is called the circadian rhythm and is synchronised by a circadian clock located in the brain. The term circadian reflects the fact that each full period or cycle is not exactly equal to 24 hours. (It is now thought we have a circadian rhythm of about 24.5 hours.) The clock alters hormone levels, digestion, body temperature, sleep and mood and is influenced by rhythmic cues from the outside world.
The cues are called zeitgebers (time givers) and they can produce both advances and delays in the body clock. The main zeitgebers are the light-dark cycle and the secretion of melatonin, although exercise also has some effect.
Research has shown that the risk of a heart attack from 6am to noon is 30-40 per cent higher than would be expected if all heart attacks occurred randomly throughout a 24-hour period. Blood pressure is at its lowest during the night but rises by 10-25 per cent between 6am and noon, placing additional stress on the wall of the coronary arteries.
Further evidence that sleep duration may affect the risk of cardiac disease comes from a number of studies showing a higher incidence of coronary heart disease in shift workers. And the lower incidence of heart disease and stroke in residents of Mediterranean countries may be linked to their habit of having a mid-afternoon snooze as well as their diet of fruit, vegetables and olive oil.
A study from the University of Athens Medical School found a positive link between having a siesta and a lower level of cardiovascular disease. Researchers followed more than 23,000 men and women aged 20-86 for six years; when they were recruited, none of the patients had any history of heart disease. They found that people who took naps had a 34 per cent lower risk of dying from heart disease than those who did not.
It’s all most interesting and suggests the effects of sleep disturbance may be adversely affecting other known cardiac risk factors. Intuitively at least, getting between seven and eight hours’ sleep per day, including daytime naps, appears to be a recipe for cardio-protection.