Oxygen deficit, also called hypoxia, in the brain is actually an absolute state of emergency and can permanently damage nerve cells. Nevertheless, there is growing evidence that to a certain extent, hypoxia can also be an important signal for growth. Together with scientists from the University Hospitals of Copenhagen and Hamburg-Eppendorf, researchers from the Max Planck Institute for Experimental Medicine in Göttingen have shown in mice that mentally and physically demanding activity triggers not only a local but also a brain-wide ‘functional hypoxia’. Although in an attenuated form, the effects are similar to oxygen deprivation. The shortage of oxygen activates, among other things, the growth factor erythropoietin (Epo), which stimulates the formation of new synapses and nerve cells. This mechanism could explain why physical and mental training have a positive effect on mental performance into old age.