Lung function dips and rises on a 24-hour cycle, reaching a peak for most people in late afternoon, researchers have found.
Lung function has a daily rhythm
Researchers from Long Island Jewish Medical Center say their work suggests this might be the best time of day to take exercise.
It might also be the time when respiratory medications are likely to have the most effect.
The study was presented at an American College of Chest Physicians meeting.
Many of the body's processes are governed by circadian rhythms which repeat in 24-hour cycles.
The new study, which only focused on the hours between 8am and 5pm, suggests that lung function may be controlled by the same pattern.
A five-year analysis of 4,835 patients found lung function was at its least effective around midday, rising to a peak between 4pm and 5pm.
Researcher Dr Boris Medarov said: "Circadian rhythms regulate our biological cycles for sleep, activity level, metabolism, and many other processes through our body's exposure to sunlight and darkness.
"Our study finds that lung function has its own rhythm that may govern how much energy we exert throughout the day and the best times to engage in certain activities.
"We often associate the end of the work day with being tired and less motivated for physical exertion; however, lung function seems to be at its best during this time.
"As a result, exercising or engaging in other physical activities in the late afternoon may help us to achieve optimal performance."
Dr Medarov said circadian rhythms of lung function may also have implications for the administration of asthma medications and the timing of medical procedures.
"Many patients with asthma and chronic obstructive pulmonary disease administer bronchodilators around the clock, when they actually may need less treatments and a different regimen that includes administering the medication at midday when their lung function is at its lowest.
"It also may be better to extubate (remove tubes) patients in the late afternoon when their lung function is at its best and breathing on their own is easier."
Dr Medarov said it might be possible to use relaxation techniques and other types of therapy to modify lung function circadian rhythms.
Penetrate the tissues
Dr Mark Britton, chairman of British Lung Foundation, said that it was well known that people with asthma showed signs of a circadian rhythm in their lung function.
However, he said they tended to reach a low in the small hours of the morning, and to reach a peak in early afternoon.
Dr Britton said it would be advisable to administer preventative medication at a time when the lungs were working most effectively, and this would enable it to penetrate as far as possible into the respiratory tissues.
However, he said many people with lung disease often required therapy at the point when their lungs were at their least effective.
He said the goal of medication was to try to ensure that any circadian rhythm was flattened out, so the lungs worked steadily all the time.
He said this was now much more achievable with the advent of longer-lasting treatments.