The simple act we all do every day – and exacts a surprising price from the body

A new Japanese study reveals that speaking aloud causes significant pupil dilation, indicating a high physiological effort comparable to intense cycling, while internal speech requires effort only in complex tasks.

A new study published in the journal iScience reveals a link between pupil size and the mode of speech—out loud or inside the head. Researchers from Japan conducted three experiments involving 80 healthy students, whose pupils were monitored while they performed various counting tasks. It was found that speaking aloud caused clear pupil dilation in all subjects, regardless of difficulty level, at a level equivalent to physical effort such as high-intensity cycling. In contrast, internal speech caused dilation only in complex tasks. The study is the first to successfully quantify the physiological difference between the two actions.