The size of the hippocampus in Japanese teenagers¡¯ brains increased during the nation¡¯s first COVID-19 state of emergency in spring 2020, suggesting their stress levels had decreased, a new study by the University of Tokyo has shown.

The research shows that, contrary to popular belief that the restrictions on academic and social activities during the pandemic added to stress among teens, their stress levels actually went down, at least temporarily. The research was , an open-access journal put out by the American Medical Association.

Researchers led by Shinsuke Koike and Lin Cai used analyzed data of about 450 teenagers between 15 and 17 in three municipalities in Tokyo who had undergone MRI scans before and after the state of emergency.

¡°There are relatively many papers on many of the effects of COVID on the brain, both before and after the actual infection (including the aftereffects). But I don¡¯t think there was any study looking at the effects of a lockdown itself, or the disruption of social life caused by it,¡± Koike said.

He added that the researchers were able to look at the effects of the state of emergency alone thanks to the fact that very few of the study participants were infected during the period when their data was analyzed.

Japan declared its first state of emergency in early April 2020. While periods varied slightly among prefectures, Tokyo¡¯s state of emergency lasted from April 7 through May 25, during which residents were asked to refrain from leaving their homes for nonessential reasons, though without penalty for disobedience. Most schools were also closed until the end of May, and even after the measure was lifted, many schools asked students to stagger attendance to avoid infections.

The average volume of the hippocampus for teenagers in the study group was about 4,500 cubic millimeters, Koike said, adding that the study showed that the participants¡¯ hippocampus size had grown by 129 cubic millimeters, at least temporarily.

The hippocampus is a part of the brain that controls memory, learning and emotions. Stressful life events such as disasters and terrorist attacks are known to lead to post-traumatic stress disorder and result in its shrinkage.

As to why the stress levels decreased during the state of emergency, Koike suspects that many high school students in Japan ¡°normally live with many stress factors.¡±

Data for the University of Tokyo research was drawn from participants of the Tokyo TEEN cohort study project, a population-based cohort study involving over 4,500 teens in the capital. Of the participants, some 450 agreed to have their MRI and other data periodically taken to track the biological changes that affect their development.