What can doctors prescribe to reduce stress in 20 minutes? Nature pills.

Time spent in nature helps people de-stress. It also offers other health benefits. But it's hard to be precise about how big a dose is needed. Or even what qualifies as time spent in nature.

Traditional medicine can work wonders, but lifestyle changes can have even more of an impact on a person's overall health.

University of Michigan researchers have recently helped out, finding that 20 to 30 minutes will do the trick for most people. As for what constitutes nature, that's pretty much in the eye of the beholder.

For eight weeks, the researchers had 36 city dwellers take a nature break of 10 minutes or more, three times a week, whenever they could fit them into their schedules. People got to choose their own location — any outdoor spot that in their opinion brought them closer to nature.

Because this was supposed to be a relaxing experience, using social media or the Internet, making phone calls, reading and aerobic exercise were all forbidden, though walking was fine. Before and after their nature experience researchers took four pairs of saliva samples from each person, measuring and comparing levels of the stress hormone cortisol.

Twenty to 30-minute nature breaks gave the most bang for the buck, significantly reducing levels of cortisol. Longer breaks continued to lower cortisol, but at a reduced rate.

“Healthcare practitioners can use our results as an evidence-based rule of thumb on what to put in a nature-pill prescription,” lead author MaryCarol Hunter, an Associate Professor in the School of Natural Resources and Environment at the University of Michigan, said in a statement. “[Our study] provides the first estimates of how nature experiences impact stress levels in the context of normal daily life. It breaks new ground by addressing some of the complexities of measuring an effective nature dose.”

Hunter points out that by letting participants decide where to enjoy nature and giving them flexibility, when it came to how long their nature break should be, “…allowed us to identify the optimal duration of a nature pill, no matter when or where it is taken, and under the normal circumstances of modern life, with its unpredictability and hectic scheduling.”

For some urban dwellers, just standing under a single tree or sitting in the sun could be the nature experience they need. Other might seek places with flowers and birds, or a calm pond with the sound of water lapping the shore.

Traditional medicine can work wonders, but lifestyle changes can have even more of an impact on a person's overall health. Doctors regularly recommend patients quit smoking or exercise more — prescribing specific step counts. Why not prescribe a nature pill, too, to help people cut down on stress?

The study appears in Frontiers in Psychology.