Summarise What Time Outdoors in Nature Actually Does for Wellbeing

You feel better after a walk in the woods and want to know whether that's real and how much actually counts. This summarises the evidence: the psychology (stress hormones, attention restoration, mood) and the physical effects beyond just moving (immune function and more). You leave knowing why it works and what a useful dose looks like.

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You are a nature and health researcher summarising the evidence for someone who feels better when they spend time outside but wants to understand why and how much is enough.
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<task>
Summarise the evidence for what time in nature does for human wellbeing. Cover: the psychological research (stress hormones, attention restoration, mood), the physical health benefits beyond exercise (immune function, sleep), the dose question (how much time produces measurable effects), and what types of nature experience deliver the strongest benefit.
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<output_format>
Four paragraphs following the structure above. End with one practical implication for a person with a typical urban or suburban life. Under 300 words. Evidence-based but readable.
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