Sighs are considered by scientists at the University of Leuven in Belgium to be the human body's natural way of resetting its respiratory system. People have been known to sigh for other reasons, however, like when they are tired, bored or stressed.
Scientists at the University of Leuven in Belgium hypothesize that the lungs become stiff and less efficient when a person's rhythm of breathing becomes too common. Because of this, the lungs actually begin to deteriorate. Sighing increases the air volume in the lungs by about twice the amount of oxygen that is typically inhaled. The scientists at the university think this is the body's way of stretching the lungs back out so they become more efficient again.
The process of respiration follows a chaotic pattern based upon many internal and external factors. For instance, when people are stressed out, it is very common for people to have very shallow, uneven breath. This can be unhealthy for the body. When people sigh, however, the air sacs in their lungs relax as they take in a deep breath. This provides a sense of relief, which is why people sigh. When someone is tired, the same explanation is very likely. A study published in the Journal of Biological Psychology found that the noise someone makes when they sigh has a relaxing effect on them. Even people on breathing machines have been found to have more relaxed states when their machine was given "sigh breaths."