Why asthma worsens at night

For hundreds of years, people have observed that asthma severity often worsens in the nighttime. Researchers have pinned down the influence of the circadian system as the reason and not sleep and physical activities as traditionally thought.

As many as 75 per cent of people with asthma report experiencing worsening asthma severity at night. Many behavioural and environmental factors, including exercise, air temperature, posture, and sleep environment, are known to influence asthma severity.

In the study, the team from Brigham and Women's Hospital and Oregon Health and Science University wanted to understand the contributions of the internal circadian system to this problem.

The circadian system is composed of a central pacemaker in the brain (the suprachiasmatic nucleus) and "clocks" throughout the body and is critical for the coordination of bodily functions and to anticipate the daily cycling environmental and behavioural demands.

"This is one of the first studies to carefully isolate the influence of the circadian system from the other factors that are behavioural and environmental, including sleep," said Frank AJL Scheer, director of the Medical Chronobiology Programme in the Division of Sleep and Circadian Disorders at the Brigham.

"We observed that those people who have the worst asthma in general are the ones who suffer from the greatest circadian-induced drops in pulmonary function at night, and also had the greatest changes induced by behaviours, including sleep," added Steven A. Shea, Professor and director at Oregon Institute of Occupational Health Sciences.

The findings are published in The Proceedings of the National Academy of Sciences.

To understand the influence of the circadian system, the team enrolled 17 participants with asthma (who were not talking steroid medication, but who did use bronchodilator inhalers whenever they felt asthma symptoms were worsening) into two complementary laboratory protocols where lung function, asthma symptoms and bronchodilator use were continuously assessed.

In the "constant routine" protocol, participants spent 38 hours continuously awake, in a constant posture, and under dim light conditions, with identical snacks every two hours.

In the "forced desynchrony" protocol, participants were placed on a recurring 28-hour sleep/wake cycle for a week under dim light conditions, with all behaviours scheduled evenly across the cycle.