People react faster when they breathe out than when they breathe in, according to a new study from Northwestern University published in iScience on September 28.

In tests with 35 volunteers, reaction times were 41 milliseconds faster during exhalation than inhalation. Responses during breathing pauses were 21 milliseconds faster than during inhalation.

The researchers stumbled on the finding while studying sleep apnea. They then designed experiments to test whether the phase of breathing affects how quickly the brain responds.

Study authors Ken Paller and Christina Yamazaki say the effect may be linked to how breathing rhythms interact with brain activity, though the exact mechanism needs more study.

The team says the result could matter for athletes and anyone doing tasks where split-second reactions count. Even small gains in reaction speed can decide outcomes in sport.

The discovery came as a surprise during sleep-apnea research, the authors said. They now hope other laboratories will replicate the result in larger groups and test whether it holds during exercise.