A German study from the Universities of Hambourg and Humbolt showed that psychological stress alters the normal cycle of hair growth.
In order to understand this study one must first understand the 4 phases of hair growth: the anagene, catagene, telogene phases and finally the expulsion of the hair.
This study used 8 weeks old mice since they are more sensitive to the stress induced in a laboratory setting. The researchers removed all the hairs from the mice so that it induced the anagene phase. Afterwards, they stressed one group of mice with multiple intense sounds at 15-second intervals for 24 hours.
Human and mice are close parents and share a very similar genome. This means that the results of this study can be extended to humans.
The results show that stress induces a premature catagene phase, the death of follicle cells and many inflammatory factors. This study is one of the first to experimentally demonstrate that psychological stress can alter the hair growth cycle and increase hair loss.