Why does asthma get worse at night?

In 1698, the British Dr. John Floyer wrote a treatise on asthma, the first major work devoted to the disease. Everything has not aged well. He warned that those who were sad or angry were more likely to suffer attacks because the sadness would stop the “Humor Movement”. He also recommended a few remedies, including regular, mild vomiting.

During an asthma attack, the airways in a person’s lungs begin to close, making it difficult for them to breathe and causing chest tightness, coughing, and wheezing. But Floyer’s article also noted another important symptom: his own asthma was almost always more severe at night, sometimes waking him at 1 or 2 a.m. Hundreds of years later, scientists found evidence to back it up: A 2005 study found that nearly 75% of people with asthma experience more severe attacks at night. A famous London hospital mortality survey in the 1970s showed that morning and night attacks were more likely to be fatal.

Yet no one really knows why asthma gets worse at night, says Steven Shea, director of the Oregon Institute of Occupational Health Sciences at Oregon Health and Science University. “Most people sleep at night, so maybe it’s sleep that’s making your asthma worse at night,” he says. Or it can be caused by the position of the body or by dust mites or allergens in the bedding. Or, Shea adds, “maybe it’s the internal biological clock.”

This biological clock is also called the circadian system. Among other important functions, it regulates hormones, heart rate, and the immune system on a cycle that lasts about 24 hours. Although this system is internal, it is strongly influenced by external factors such as light and dark, meal times and work schedules.

Historically, it has been impossible to isolate the role of the circadian system from people’s behavior and environmental risks “because they go hand in hand,” says Frank Scheer, director of the medical chronobiology program at Brigham and Women’s Hospital. “You can’t tell what is actually causing the changes in lung function. But in an article published this month in Proceedings of the National Academy of Sciences, a team led by Scheer and Shea finally found a way to separate the circadian system from any external factors that could be contributing to asthma.

First, they asked their 17 study participants, all of whom were previously diagnosed with asthma, to monitor their lung function at home as they go about their daily lives. Four times a day, the participants used a hand-held spirometer to test how much air they could expel from their lungs in one second, a measurement called FEV1. (The more, the better.) They also recorded their symptoms and noted when they should use their rescue inhalers.

Then things got considerably more complicated. The same group of participants were subjected to two different experiences while living in dimly lit rooms at the Center for Critical Inquiry at Brigham and Women’s Hospital. In one experiment, called the “constant routine protocol,” participants stayed in bed for 38 hours without being allowed to sleep. They could not get up to use the bathroom or do strenuous activities. Every two hours they ate the same snack, a little peanut butter and jelly or tuna sandwich. They were allowed to listen to books on tape, chat with nurses, or play card games, but they could not move, get excited, or get angry.

In those rooms without a clock or window, and with subjects no longer tied to their daily work or home schedule, the weather outside felt like it didn’t exist. Participants had no idea when the sun was rising or setting, when it might be lunchtime, or when they should fall asleep.