Forty Winks

4 June 2008

The metabolic and endocrine hormonal changes resulting from sleep deprivation have a similar impact on the body to the ageing process and may increase the severity of age-related chronic disorders such as heart disease and diabetes, according to researchers at the US Department of Medicine in Chicago. Over the last hundred years, the average number of hours people spend asleep per night in the more developed countries has decreased from nine hours to seven-and-a-half hours. It is thought eight hours is the amount of sleep the average person requires for the body to be fully rested.

Sleeping for an hour after lunch improves day long performance of demanding tasks more than just a half hour siesta, and definitely more than working right through. In a study at Harvard University, researchers set volunteers an exhausting concentration task for one hour each time, four times through the day. The performance of those who did not sleep after lunch deteriorated as the day progressed, the half hour sleepers" performance stayed more or less the same, while that of the hour-long nappers improved. (New Scientist May 26th 2002).

A Chinese newspaper has warned its readership that intensive mental work straight after lunch can cause dizziness, insomnia and even increase the danger of a stroke. The warning in the Health Times reflects the fact that the traditional after-lunch nap, or xiuxi, is coming under increasing pressure. In previous decades the topic might never have arisen, but as China's market reforms take effect, the after-lunch nap is an institution under threat. Now China's doctors are coming to its rescue. The latest edition of Health Times describes the case of a 30-year-old man working for a private enterprise. He sought help complaining of dizziness, insomnia, weakness and memory loss. Doctors diagnosed that he was suffering from the effects of long-term intensive mental work - especially just after meals. The Health Times also warned that a thickening of the blood which occurs after eating can combine with certain other factors to increase the risk of strokes. Whether such warnings will cause China's increasingly competitive workforce to rush to their bosses and claim an old fashioned xiuxi is not clear. Among other recent health tips in various dailies are an item warning women against wearing tight cotton knickers and a two-page article giving advice about how to live to be 100. (Reported in BBC News July 2002)


Afternoon naps - or siestas - are practiced in many countries  of the world including China, Spain and Arabia. They are typically short naps or rest periods of no more than an hour taken in the afternoon. Several recent studies have shown an inverse relationship between siesta taking & fatal heart attacks. In a recent epidemiological study of 23,000 people in Greece, those who regularly took siestas showed a 37% reduction in coronary mortality compared to those who never nap, while individuals who occasionally napped in the afternoon had a reduction of 12%.

Why do afternoon naps affect cardiovascular function? One reason could be changes in blood pressure. At night, our blood pressure and heart rate decreases as we sleep. Some researchers hypothesize that the lower blood pressure reduces strain on the heart and decreases the risk of a fatal heart attack. Most studies have focused on cardiovascular behavior in nighttime sleeping. However, a new study entitled "Acute Changes in Cardiovascular Function During the Onset Period of Daytime Sleep"(1) provides a detailed description of changes in cardiovascular function of daytime sleep in healthy individuals, comparing napping with other daytime activities such as standing and lying down without going to sleep.

Nap versus Conscious Rest
The researchers tested nine healthy volunteers (eight men, one woman) who did not routinely take afternoon naps. The volunteers attended the university laboratory on three separate afternoons after sleeping four hours the night before. The volunteers wore equipment that checked blood pressure, heart rate, and forearm cutaneous vascular conductance (which determines dilation of blood vessels). During one afternoon session, the volunteer spent an hour resting, lying face-up in bed. During another session, the volunteer spent an hour relaxed, but standing. And in one session, the volunteer was allowed an hour to sleep, lying face-up. During the sleep stage, the researchers measured the volunteer's different stages of sleep. The session in which the volunteer was allowed to fall asleep was delineated into three phases:

Phase 1: A five-minute period of relaxed wakefulness before lights were turned off (volunteers had been lying on the bed for a minimum of 15 minutes before this phase).
Phase 2: The period between "lights out" and the onset of Stage 1 sleep (loss of some conscious awareness of the external environment).
Phase 3: The period between the Stage 1 and the onset of Stage 2 sleep (conscious awareness of the external environment disappears).

Changes Found Only in Pre-Sleep
Researchers found a significant drop in blood pressure during the sleep trial, but not during the resting or standing trials. What's more, this drop in blood pressure occurred mostly after lights out, just before the volunteer fell asleep. This reduction in blood pressure may be one explanation for the lower cardiovascular mortality that some studies have found among people who habitually take siestas. On the other hand, some studies of nocturnal sleep have shown that blood pressure rises when we awake and that more cardiac deaths occur in the mornings. So the John Moores team will next look at blood pressure during the waking portion of the afternoon nap to see if this period may also pose an increased danger of coronary mortality.

(1) Acute Changes in Cardiovascular Function During the Onset Period of Daytime Sleep: Comparison to Lying Awake and Standing, found in the online edition of the Journal of Applied Physiology, published by The American Physiological Society. The study was conducted by Mohammad Zaregarizi, Ben Edwards, Keith George, Yvonne Harrison, Helen Jones  and Greg Atkinson, of the Liverpool John Moores University in Liverpool, U.K.

: Lifestyle

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