University students in the United States are more stressed out than ever before, and to help them cope, hundreds of colleges across the country have implemented animal visitation programs.

These real-life petting zoos are thought to alleviate stress in undergraduates, but so far, there's been scant scientific evidence that such programs actually work. Looking at the major stress hormone, cortisol, a new study is one of the first to delve into the physiological benefits of petting pets on campus.

"We already knew that students enjoy interacting with animals, and that it helps them experience more positive emotions," says human development researcher Patricia Pendry from Washington State University.

"What we wanted to learn was whether this exposure would help students reduce their stress in a less subjective way."

Using salivary cortisol levels as an indication of academic stress, the study focused on 249 college students, who were randomly split into four groups during a real university animal visitation program.

For ten minutes, the first group of students was allowed to pet, play and just generally hang out with a group of cats and dogs from a local shelter. The other groups were not so lucky.

While the second group spent the full ten minutes in line, waiting and watching other students interact with the animals, the third group didn't even get that far, and was only shown images of the animals in a slideshow.

The last group was simply put on an indefinite waitlist, meaning they had no visual or physical exposure to the animals, even though they were told they would get to see them soon.

Salivary samples were taken by each participant three times throughout the day, once upon waking, and two more times 15 and 25 minutes after the 10 minute experiment.

As the first animal visitation study to examine salivary cortisol outside of a laboratory setting, the results are promising. In the end, the first group showed significantly lower levels of salivary cortisol than all the other groups. And that remained true no matter what a student's cortisol levels were upon waking, the time they'd spent awake that day, or their circadian rhythms.

In short, this suggests that just 10 minutes of petting time can have a significant impact on a student's physical stress levels. And the authors are hopeful that this could assist universities in determining the best type of interaction and dosage for their animal intervention programs.

While we must keep in mind that the sample size of this study is rather small, the results complement extensive research on animal-assisted therapy in general, which has shown that having a dog is beneficial for both human health and wellbeing.

Touch is one of the most powerful ways to bond with your animal and it doesn't take much scratching or stroking to see the difference. One study found that cortisol levels were significantly decreased after 15 and 30 minutes of an owner stroking, petting and talking with their labrador.

While another study from 2017 found that a 15-minute pet therapy session almost immediately reduces a student's psychological stress and their blood pressure.

The team from WSU suggests that more research should be done on the hypothalamic-pituitary-adrenal (HPA) axis, which is one of the body's most sensitive stress systems. They think animal visitation programs may settle down this system through modulation of the hormone oxytocin.

"Given that one-on-one and group interactions with animals have reduced individuals' cortisol levels in therapeutic and health care settings in the past, our results add to the evidence for the efficacy of brief, universal, university-based animal visitation programs to reduce university students' physiological stress," the authors conclude.

The research has been published in AERA Open.