The Physiological Sigh

The physiological sigh method consists of a breathing pattern consisting of two inhales through the nose followed by an extended sigh (exhale) through the mouth. The first breath though the nose is almost to capacity, followed by one more quick nasal inhale that further opens the air sacks in the lungs.

You can check out a video here by Andrew Huberman from Huberman Lab explaining the inner workings of breathing and physiological sigh. The key message here is that if you want to calm yourself down, you need to emphasise your exhales.