Researchers at Rutgers University suggest that recent changes in people's lifestyles have caused more stress. Stress causes the body to experience physical changes such as a faster heartbeat and breathing rate. The right level of stress can make a person feel stronger and more energetic. Researchers believe that in the past this energy help people to hunt for animals and to protect themselves from danger (cannon, 1932). They released stress with physical activity involved in hunting, and then their bodies could relax again. Although people today still experience an increase in energy in response to stress, they do not use energy like the hunter-gatherers did. As a result, it is more difficult for their bodies to let go of stress.