A two-hour, weekly dose of "nature" significantly boosts health and feelings of well-being, a study in England concludes, even when you're just sitting back and enjoying the serenity.

The physical and mental health benefits of time spent in the park, woods or at the seashore are well known, but the new study is the first large-scale investigation of how much time is needed to have an effect. If future research confirms its findings, the two hours in nature could join the already official health recommendations of five fruit and vegetables a day and 150 minutes of exercise a week.

The result is based on interviews with 20,000 people in England about their activities over the past week. A quarter of people who spent little or no time in nature complained of poor health, and almost half expressed dissatisfaction with their lives - a standard measure of wellbeing. In contrast, only a seventh of people who spent at least two hours in nature reported poor health, while a third reported being dissatisfied with their lives.

"It struck us that this was true of every single group we could think of," says Dr Matthew White, the study's leader from the University of Exeter Medical School. The benefits of the two-hour dose were the same for old and young, rich and poor, city people and country people, he said.

This is also true for people with long-term illnesses and disabilities, White says. "Getting out in nature proves to be beneficial for everyone in general. You don't need to exercise - you can just sit on the seashore."

The researchers were also surprised that it didn't matter whether the two hours in nature were taken all at once or through a series of shorter visits, or whether people went to the city park, the woods or the seashore.

The study, published in the journal Scientific Reports, used data from the Natural England survey, the world's largest study of people's weekly contact with the natural world. It does not include the time people spend in their gardens as this is not measured. But White says half of people see their gardens more as a duty than a pleasure. The data shows that two hours is the threshold for getting positive impacts: spending much more time than that in a natural environment does not lead to additional benefits.

"I'm constantly surprised by the magnitude of these effects," says White. The health benefit was the same as what previous research had indicated in terms of doing exercise according to recommended levels or living with wealthier neighborhoods versus poorer areas.

The study does not attempt to reveal why contact with nature is so beneficial, but White suggests that a significant factor is a sense of calm: "Most people are under multiple stresses at any given time. So if you retreat to a natural environment, it's quiet, relaxing and you get time to start rethinking things."

"Increasingly we are finding that the richness of biodiversity in an environment is important. We tracked the nature outings of 4,500 people from the same study and found that their stress levels decreased most sensitively if the place featured a place of outstanding natural beauty, a place of special scientific interest, or something similar."

The researchers took into account a series of factors to reach their conclusions, including the degree of landscaping in the people's neighborhoods, air pollution levels and whether they were married, had children or dogs.

They did not fully take into account whether health benefits were due to increased physical activity. But the researchers wrote: "Research on shinrin-yoku - Japanese "forest bathing," for example - suggests that significant psycho-physiological benefits are derived from simply sitting passively in a natural versus an urban environment."