Asked by: Alexandra Missoni, Edinburgh
Sighing is a spontaneous deep inhalation and exhalation. It serves to boost blood oxygen levels and fully reinflate the lung tissue. Sighing is not contagious in the same way as yawning, which suggests that its primary function is to do with respiration rather than communication of any state of mind. But it’s also associated with certain moods, and laboratory rats have been shown to sigh with relief.