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.