In an hour-by-hour analysis, patients we suspect to have asthma are significantly more likely to have positive bronchodilator responses early in the morning; with each passing hour before testing, there was small decrease (8%) in positive response. Patients also were more likely to have positive responses in the winter. 

Knox-Brown B et al. Effect of time of day and seasonal variation on bronchodilator responsiveness: The SPIRO-TIMETRY study. Thorax 2025 Mar 11; [e-pub]. (https://doi.org/10.1136/thorax-2024-222773)