Ever wonder how the placebo effect works? On the surface, the effect of the 'dummy pill' makes no sense. How can someone suffering from an infection miraculously recover just as well, whether they take a medicinal drug or a sugar pill with no curative value?

There may be an evolutionary explanation for the placebo effect, according to a new finding that suggests that the immune system has a mind-controlled on-off switch to save energy and that people are able to heal themselves.

Scientists questioning why people need to wait for the placebo before the recovery process from an infection begins have now found something similar to the placebo effect in animals.

Researcher studied Siberian hamsters and found that the rodents' bodies did little to fight infection if the lights above their laboratory cages mimicked the short days and long nights of winter, but mounted a full immune response if the lighting was changed to replicate summer conditions, according to a study published in the journal Annals of Family Medicine.

Peter Trimmer, a biologist at the University of Bristol, and his team believe that, similar to the response to lighting in Siberian hamsters, people who think they are taking medicine to treat an illness, but are actually receiving a placebo, exhibit an immune system response that is twice as active as people who take no pills.

Trimmer and his team built a computer model that supported past findings that intervention triggers a mental response which kick-starts the immune response.

Because the immune system takes up a lot of the body's energy when it is in action, an animal's energy reserves could be severely exhausted if the immune system launches a long response to an illness, the model suggests that if the infection is not likely to cause death, it would be better for the body to wait and see whether fighting the illness will put the animal in other danger.

The model, published in the journal Evolution and Human Behavior, suggests that animals that live in more challenging environments where food is scarcer live longer and reproduce more offspring if they endure infections without deploying an immune response.

However, researchers found that animals living in more favorable conditions where there is more access to food were better off launching an immune response immediately so they can return to health quicker, suggesting that there was a clear evolutionary benefit to switching the immune system on and off depending on environmental conditions.