Lungs lose power at lunch time
18:30 26 October 04 NewScientist.com news service
Human lungs experience a dip in performance around noon and not just at dawn as previously thought, suggests a large study into the effect of circadian rhythms on lung function.
The findings suggest doctors could lower drug doses to respiratory patients by administering the drugs shortly before noon, when most needed.
"Almost every single physiological process follows certain circadian rhythms," says study author Boris Medarov, a physician at the Long Island Jewish Medical Center in New Hyde Park, New York, US. Sleep is the most obvious example of a process following a regular, 24-hour cycle, but hormone levels controlling growth, metabolism and the immune system follow a daily pattern too.
Various health problems also tend to peak at certain times of day. Heart attacks strike most often in the morning, while night-time sees the most asthma attacks.
Studies of asthma patients show the worst symptoms are most likely between 0400 and 0600, before improving steadily during the day. Symptoms are least evident around 1700 before intensifying again at night.
Medarov studied the lung capacity of about 4800 patients, plotting each measurement against the time at which the test was taken, between 0800 and 1700. He found that lung function actually dips at noon before peaking in the late afternoon.
His results show a 15% to 20% difference in lung performance between noon and late afternoon. However, healthy people have such strong lungs that such a change would hardly be noticeable, he says.
"That difference would basically apply to people with lung disease - asthma, chronic bronchitis or emphysema," he told New Scientist. "Considering this information, they may better time their medication, which theoretically may lead to less use. Also, the late afternoon may be the best time for those people in rehab programs to take exercise."
Medarov presents his study on Wednesday at a meeting of the American College of Chest Physicians in Seattle, Washington. He admits that he cannot disprove previous studies that show lung function is worst around dawn, since his study was limited to working hours.
And his research made no allowance for health of its participants, so its applicability to people with asthma or other specific disorders is unknown.
"If for some reason the healthy people came at 5pm and the sick people came in the morning, that would fit this result," says Nizar Jarjour, a pulmonary specialist at the University of Wisconsin in Madison, US.
"But the idea that lung function does change during the daytime - that's interesting," Jarjour told New Scientist. He says changing patients' medication times can indeed affect both a medicine's effectiveness and its side effects.
But he adds that doctors must consider the timing of drugs on a case-by-case basis, making sure asthmatics' doses cover night-time, when previous studies have shown attacks peak.
Jarjour says hormones are probably responsible for the lungs' circadian rhythms, though when people lie down at night, the horizontal orientation could also inhibit lung function. "But the very interesting unanswered question is why human beings were built to have better lung function in the afternoon hours," he says.