Nearly half of Americans spend less than two hours outdoors during an entire week, according to a national survey from the Nature & Health Alliance involving researchers at Texas A&M University. California physician Dr. John La Puma warns that this indoor routine may carry overlooked health consequences.

La Puma, an expert in nature based medicine and author of “Indoor Epidemic,” says humans spend about 93% of their lives inside. He describes the consequences as a “hidden health tax” and proposes a 7% outdoor prescription as a remedy.

Many people may not connect their exhaustion with the amount of time they spend indoors, La Puma told Fox News Digital. Some remain tired after a full night of sleep, rely on coffee to feel normal, crash during the afternoon and struggle to fall asleep at night despite normal laboratory results.

“And they come into the office saying, ‘Why do I feel so exhausted?’ It's the four Bs,” he said. His concerns include disruption of the body clock, burnout, afternoon brain fog and metabolic effects associated with an altered cortisol rhythm.

Outdoor light can be many times brighter than typical indoor lighting, making it a powerful signal for the body’s circadian rhythm, according to La Puma. Without that signal, he said, the body may struggle to recognize when it should sleep.

Getting bright outdoor light soon after waking can help synchronize the body’s sleep and wake cycle and support the natural rise in melatonin later in the evening, La Puma said. He recommends spending about 10 minutes outside within the first hour after waking, without wearing sunglasses.

La Puma also uses the term “ultraprocessed time” for hours consumed by screens, scrolling and other digital distractions. “Ultraprocessed time is kind of like ultraprocessed food,” he said. “It’s not nutritious. It’s synthetic. It’s artificial. It’s ubiquitous.”

Excessive indoor time can produce an overload of screen exposure because people may use the same devices for work, entertainment and relaxation. La Puma said the body may not interpret that screen use as genuine rest or escape.

Indoor environments also limit how far people can see. “And equally important, no horizon. We are horizon-deprived as a species because indoors, we can't see more than 20 feet in front of us,” La Puma said.

“Your eyes need distance like your lungs need air,” he added. To combat burnout, he recommends getting up once every hour, walking to a window and looking outside for one minute.

“It relaxes your eyes, relaxes the muscles around your temples, reduces headache if you have it, improves energy and reduces fatigue,” he said. “That's the beginning of being able to reverse burnout.”

Poorly ventilated indoor spaces may create another problem by allowing carbon dioxide to accumulate. Indoor CO₂ levels can rise above 1,000 parts per million, and some research has linked elevated levels with reduced cognitive performance.

La Puma said people who remain indoors all day may feel mentally foggy by the afternoon, particularly in spaces with inadequate ventilation. “So it's not that you're just making bad decisions in the afternoons — there's a biologic reason for it,” he said.

Getting outside allows the eyes to focus at greater distances while offering relief from potentially stuffy indoor air. The EPA notes that concentrations of some pollutants can be two to five times higher indoors than outdoors.

An out of sync body clock can also disrupt the body’s normal cortisol rhythm, La Puma said. Research has linked altered cortisol activity with greater abdominal and visceral fat, although the relationship is complex.

“Again, the way to improve your metabolism is to get daylight at the right time, in the right place, for the right duration — early in the morning especially,” La Puma advised. He said this produces an early cortisol increase that supports wakefulness before the hormone drops appropriately.

La Puma distinguishes intentional outdoor exposure from the incidental time people accumulate while walking between a car and an office. He recommends spending at least 17 minutes a day intentionally outdoors in a blue or green space.

For people in large cities who cannot see the sky, he suggests focusing on a distant rooftop. When going outside is not possible, looking through an open window can help the eyes focus beyond the confines of an indoor space.

“Be intentional with your time — look at the sky in the morning … to reset your body clock and watch the clouds,” La Puma advised. He said 17 minutes daily is the minimum effective dose needed to begin addressing the four Bs.