Have you ever frozen when you heard a strange sound at night? This is a natural reaction. Many animals, like rabbits, stop moving when they sense danger.

Moving things are easy to see, so stopping helps them hide and watch. Humans do this too. When we freeze, our heart beats more slowly. In one study, people's bodies moved less when they looked at pictures of angry faces.

When we suddenly sense danger, we often freeze before we fight or run away. Many animals do the same. Since predators notice movement, staying still helps an animal avoid attention while it watches the threat. A Dutch researcher explains that freezing is not doing nothing but a brake on movement.

During fight or flight, the heart speeds up, but during freezing, the heart rate drops. In a 2010 study, 50 women stood on a platform that measured tiny body movements and looked at pictures of angry, neutral and happy faces. Only the angry faces reduced their body sway and heart rate. The results suggest that humans also have a built-in freeze response.

Fear is often associated with fight or flight, but many species respond to danger first by freezing. Because predators are highly sensitive to movement, remaining still reduces the chance of detection while allowing careful observation. According to a 2017 review by a Dutch neuroscientist, freezing is not passivity but a brake on action: whereas fight or flight is driven by the sympathetic nervous system and raises heart rate, freezing involves parasympathetic dominance and a slowing of the heart. In animals, signals from the amygdala to a brainstem region called the periaqueductal gray trigger freezing.

Humans show a similar pattern. In a 2010 study, 50 women stood on a platform that recorded body sway while viewing angry, neutral and happy faces; angry faces alone reduced sway and heart rate, and the effect was related to how anxious the participants felt. A 2015 psychiatric review described a defense cascade ranging from arousal through fight or flight and freezing to tonic and collapsed immobility. But the link between human and animal freezing is largely a functional analogy, and matching extreme immobility to experiences such as legs giving way lacks direct evidence.