Researchers measured cortisol in people's saliva before and after time in nature. The sweet spot was shorter than most of us assume.
When I first read the cortisol study I expected the answer to be an hour. It was 20 minutes.
Cortisol is the hormone your body releases under sustained pressure. It is useful in the short term and corrosive when it stays high all day. Several research groups have now measured it in saliva before and after time in nature, and the pattern is consistent: it drops, it drops fastest early, and you do not need to be doing anything athletic for it to happen.
Hunter and colleagues (2019) asked 36 city residents to spend at least 10 minutes, three times a week, somewhere outdoors that felt natural to them. The setting was their choice: a park, a yard, a tree-lined street. Over eight weeks they gave saliva samples before and after. Cortisol fell during the visits, and the efficiency of the drop peaked between 20 and 30 minutes. After that it kept falling, but more slowly. Whether people sat or walked made no measurable difference.
Olafsdottir and colleagues (2020) ran a randomized trial with 90 university students in Iceland, some tested during exam season, some in a calmer term. Each did 40 minutes of one of three things: walking in nature, walking on a treadmill in a gym, or watching nature on television. Cortisol fell in all three groups. Mood improved most in the group that walked outside, and the difference was largest for the students who were under real exam stress at the time.
Sudimac and colleagues (2022) scanned 63 people before and after a one-hour walk, either through the Grunewald forest in Berlin or along a busy shopping street. Activity in the amygdala, the region that flags threat, decreased after the forest walk and stayed level after the city walk. The urban walk did not make things worse. The forest walk made them measurably better.
Gidlow and colleagues (2016) had 38 adults walk for 30 minutes in three settings on different days: a residential street, a green space, and a green space with water. Mood and cortisol improved in all three. The green and blue settings produced stronger feelings of restoration and improvements in a cognitive test that were still present 30 minutes after the walk ended.
Antonelli and colleagues (2019) pooled 22 studies of shinrin-yoku, the Japanese practice of slow time in forests, and found a consistent drop in cortisol. They also flagged a problem: in several studies the forest group already had lower cortisol before the session began, which suggests expectation plays a part. Expectation is still a real effect. It is just worth knowing it is in the mix.
The cortisol study is the best evidence I know that sitting works. Participants who sat on a bench got the same drop as those who walked. If walking is off the table, a bench under a tree for 20 minutes is the whole protocol.
No. The 20-minute study let people pick their own spot, and most chose ordinary urban greenery. The forest walk in Berlin produced a bigger brain change than a street, but a park sits somewhere in between.
Partly. In the Iceland trial the treadmill group's cortisol also fell. What the outdoor group gained on top was the mood shift, especially under stress.
Within the session. Samples taken right after a 20-minute visit already showed the drop.
For a guided version built around exactly this length, Ease & Ground in the City is 35 minutes in Central Park, written with a therapist.
On the science: every study named here is real and linked below. The summaries and takeaways are my own reading of them, simplified to be useful rather than complete. Where a detail matters to you, the paper is one click away.
