Taking just 20 minutes out of your day to stroll or sit near nature will significantly lower your stress hormone levels, a new study suggests.Healthcare practitioners can use this finding to prescribe 'nature pills' to have a real measurable effect, according to researchers from the University of Michigan."We know that spending time in nature reduces stress, but until now it was unclear how much is enough, how often to do it, or even what kind of nature experience will benefit us," said lead author MaryCarol Hunter from the varsity.For the study, published in the journal Frontiers in Psychology, the research team involved 36 participants. Over an eight-week period, they were asked to take a 'nature pill' for at least 10 minutes, three times a week. Levels of cortisol -- a stress hormone -- were measured from saliva samples taken before and after taking the 'nature pill', once every two weeks.The data revealed that just a 20 minute nature experience was enough to significantly reduce cortisol levels. And if you take in a little more nature experience - 20 to 30 minutes sitting or walking - cortisol levels dropped at their greatest rate, the researchers said."Our study shows that for the greatest payoff, in terms of efficiently lowering levels of the stress hormone cortisol, you should spend 20 to 30 minutes sitting or walking in a place that provides you with a sense of nature," Hunter noted.