Image copyright Getty Images Image caption Therapy dogs are used in more than 1,000 universities and colleges in the US

Stress among students really can be reduced by spending time with animals, according to research from the US.

It has become increasingly common for universities to bring "therapy dogs" on to campus - but claims about their benefits have often been anecdotal.

Now, scientists say they have objective evidence to support the use of dogs.

Patricia Pendry, from Washington State University, said her study showed "soothing" sessions with dogs could lessen the negative impact of stress.

Image copyright Getty Images Image caption Dogs are also used to help people with post-traumatic stress disorder

The study of more than 300 undergraduates had found weekly hour-long sessions with dogs brought to the university by professional handlers had made stressed students at "high risk of academic failure" or dropping out "feel relaxed and accepted", helping them to concentrate, learn and remember information, she said.

Image copyright Getty Images Image caption A children's hospital in California got its first therapy dog this year

"Students most at risk, such as those with mental health issues, showed the most benefit," said Dr Pendry.

Image caption The dog therapy research team at Washington State University

The study was supported by the Mars group, which as well as funding animal research, includes chains of veterinary surgeons, pet hospitals and pet-food brands.

In the US, about 1,000 campuses are using therapy pets.

It has also become more common in the UK, with Buckingham, University College London, Cambridge, Nottingham Trent, London Metropolitan and Swansea among those deploying dogs.

The University of Middlesex has even put "canine teaching assistants" on to the staff, to stop lonely students dropping out.

Image caption The university study involved 300 undergraduates at Washington State

Previous research has suggested stroking pets can reduce stress hormone levels.

Image caption Students spent an hour with dogs, brought to the university by professional handlers

"There does seem to be something specific about the reducing of anxiety from the petting of animals," said Dr Pendry.

Image caption Middlesex University has put dogs on the staff as "canine teaching assistants"

"Do we fully understand the mechanism? No," said Prof Nancy Gee, a psychologist at the State University of New York and researcher from the Waltham Centre for Pet Nutrition, also part of the project.

But students appeared to "feel calmer and more socially supported", giving them more confidence in their studies.

Even just looking at animals could sometimes lighten the mood, Prof Gee added.