Teenagers staying up late is already a concern for many parents, since it can affect things like mood and academic performance the next day. Now new research has suggested another reason why a late bedtime could be bad for teens’ health, after finding that teenagers who prefer to stay up later and wake up later the next day may be more likely to have asthma and allergies compared to those who head to bed earlier. Led by researchers at the Barcelona Institute for Global Health, Spain, the new study looked at 1,684 teenagers aged 13 and 14 who were living in West Bengal, India, and taking part in the Prevalence and Risk Factors of Asthma and Allergy-Related Diseases among Adolescents (PERFORMANCE) study. As part of the study, the teens were asked to report if they had ever experienced or were currently experiencing any respiratory symptoms such as wheezing or asthma, or symptoms of allergic rhinitis, such as a runny nose and sneezing. They also answered questions on their sleep habits, such as what time at night they tend to feel tired, what time they choose to wake up, and how tired they feel first thing in the morning to determine their chronotype, which is an individual’s preferred time to sleep and to be active. Based on their responses, the teenagers were determined to be an “evening type,” a “morning type” or an in-between “intermediate” type. The findings, published in ERJ Open Research, showed that the teens who were evening types appeared to have a three times higher risk of currently having asthma and a two times higher risk of currently having rhinitis, compared to the morning types who went to bed earlier and got up earlier. Although the link wasn’t as strong for intermediate types, these participants also showed significantly higher risks than the morning types. The findings also held true even after the researchers had taken into account other known risk factors for asthma and allergies, such as where the participants live, whether they were exposed to secondhand smoke due to family members smoking and whether they had a pet. Although previous research has already found strong links between asthma symptoms and the body’s internal clock, the researchers say that this is the first study to look at how being a morning or evening person can affect the risk of asthma in teens, and provides further evidence that sleep timing is important for teenagers’s health. “We can’t be certain that staying up late is causing asthma, but we know that the sleep hormone melatonin is often out of sync in late-sleepers and that could, in turn, be influencing teenagers’ allergic response,” said lead author Dr Subhabrata Moitra, who is now at the University of Alberta, Canada.