Modern humans spend approximately 93% of their time indoors. This is an evolutionary novelty — for 99.9% of human existence, we lived in direct, continuous contact with natural environments. The consequences of this disconnection are now measurable: increased rates of myopia (from lack of outdoor light), vitamin D deficiency, disrupted circadian rhythms, and elevated baseline stress.

Richard Louv coined "nature deficit disorder" in 2005. While not a clinical diagnosis, it describes a documented pattern: the less time people spend in natural environments, the higher their rates of anxiety, depression, attention difficulties, and chronic stress.

Attention Restoration Theory (Kaplan, 1995): natural environments allow "directed attention" to rest while engaging "involuntary attention" — the mental equivalent of active recovery. Studies show 20 minutes in nature significantly improves cognitive performance and reduces mental fatigue.

Japanese forest bathing (shinrin-yoku) research demonstrates that 2 hours in forested environments reduces cortisol by 12.4%, reduces blood pressure, increases natural killer cell activity (immune function), and shifts autonomic balance toward parasympathetic. These effects persist for days after exposure.

The "20-5-3" framework synthesizes the evidence: 20 minutes in a natural setting three times per week for mood regulation, 5 hours per month in semi-wild nature for creativity and problem-solving, 3 days per year in wilderness for deep psychological reset.

Nature exposure isn't equally accessible. Low-income urban neighborhoods have 42% less tree canopy than wealthy ones. This creates a health equity issue: the people with the highest stress levels have the least access to nature's stress-reducing effects. Biophilic design — integrating natural elements into built environments — attempts to address this through indoor plants, natural light, water features, and natural materials. The evidence is promising but preliminary.