Robert Sothern is the ultimate quantified man. For 48 years, five or six times each day, he's been tracking his bodily functions and mental acuity in extensive — almost absurd — detail.

"I measure my heart rate for a minute," he tells me on a recent phone call. "I measure blood pressure. I measure respiration for two minutes, breathing slowly and counting how many times I inhale. I measure peak flow [air flow from the lungs], and I'm doing time estimation [counting to 60 without looking at the clock, and then seeing if it matches]." Fourteen years ago, he started to wear a step tracker.

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The former chronobiologist at the University of Minnesota still rarely misses a day, and at 69 years old he says he'll continue making measurements for the foreseeable future.

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In the 1960s, Sothern was a volunteer for circadian rhythm experiments. (A circadian rhythm is simply any body process that varies throughout the day and repeats itself daily.) When he was 20, researchers kept him in isolation for three weeks, with no windows or time markers, just to see what would happen to his vital signs and his perception of time.

The results were quite interesting: His 21 days in the chamber felt, to him, like 18 days, which suggested that our internal clocks run on cycles longer than the 24-hour standard day. (Sothern was surprised when the test was over and asked to stay in isolation just a few more hours to finish a book he was reading.)

That experience got him wondering: Our body's sleep patterns, hunger, and so on fluctuate throughout the day according to circadian rhythms. Are there similar patterns that reveal themselves over decades? Sothern started keeping meticulous records of his own bodily functions to find out. "It got to be four years, five year, then 20 years, and now it's 48 and a half years," he says. Once he hits 50 years, he hopes to publish a summary of his circadian life.

So what can we learn from his records? Germaine Cornelissen-Guilaume, a fellow chronobiologist at the University of Minnesota, has analyzed some of Sothern's data, and it supports a few findings. For one: As we age, our internal circadian clock appears to break down.

Consider blood pressure, which usually follows a circadian rhythm: It gets lower at night and goes higher during the day. But as we get older, that changes. "The decrease at night is less, and the increase during the day is less too," Cornelissen-Guilaume says. She says — generally, from other people's data as well — that pattern is consistent across several organ systems.

Curiously, this phenomenon is also similar to what happens when researchers destroy the suprachiasmatic nucleus — essentially, the brain's master clock — in mice. This suggests that aging is related to the breaking down of the body's master clock.

Sothern's data can also reveal more subtle changes that happen to our daily and yearly cycles as we age. "A longitudinal [data set] — even if it’s only from a single individual — can tell you an awful lot of information that you cannot obtain from large groups at a single time," Cornelissen-Guilaume says. "This is a life commitment; this is really unique."

Sothern is now retired from the university, where he studied whether the timing of cancer drugs impacted their efficacy or diminished their side effects. His retirement schedule allows him to take the measurements at home. But in the past, he used to carry around a briefcase or backpack with his recording equipment.

He says the project isn't an obsession (he did have his parents measure their blood pressure twice a day for decades as well), but a pursuit of science. One day aliens may come down to Earth, and if they want a thorough record of the physiological patterns of humankind, they'd might want to check out Sothern's 3,100 pages of personal records, stored neatly in three-ring binders, in the spare room of his house in St. Paul, Minnesota.

"We are rhythmic creatures," Sothern, says. "You look at this ... data [and] you can see 10-year cycles in it. You can see daily cycles. You can see even men have something approaching a 28-day cycle in their beard growth — which I did measure for three years too. By having a rhythm, it proves that you are alive."