Sleep apnea — repeated pauses in breathing during sleep — is much more common than previously thought.

The condition increases the risk of high blood pressure, depression, heart disease, cancer and death. Losing weight and exercising can offset it. People who sleep too little or too much, regardless of whether they have sleep apnea, are more likely to be overweight.

Those and other findings about sleep are common knowledge among scientists today thanks to Don Chisholm, Mary Ellen Havel-Lang, Paul Minkus and more than 1,540 other participants in the Wisconsin Sleep Cohort Study.

The UW School of Medicine and Public Health study, of state workers who periodically undergo sleep tests at UW Hospital and provide other information, has continued for 23 years.

Its latest splash in the national headlines came last month, with a finding that people with severe sleep apnea are five times more likely to die from cancer. Inadequate oxygen during sleep might promote tumor growth, researchers said.

As the longest-running assessment of sleep of its size in the country, the Wisconsin study has reshaped notions about sleep and its effects on overall health, said Michael Twery, director of the National Center on Sleep Disorders Research.

Snoring, which once symbolized peaceful sleep, now is a sign of possible breathing troubles, said Twery, whose center is part of the National Institutes of Health, which funds the $1.5 million-a-year Wisconsin study.

Also, Twery said, "the study is showing that if you have sleep apnea over a period of time, it erodes your health."

Wired for sleep

Chisholm, 75, who retired three years ago as a UW-Madison electrician, said he's had seven overnight sleep evaluations through the study. Researchers place sensors on the scalp, chest, hands and legs and near the eyes, nose and mouth. The devices measure breathing, heart rhythm, movement and stages of sleep.

"It's kind of disconcerting to have all of these wires hooked up to you," said Chisholm, of Madison. "But I've gotten used to it."

He's learned, from the reports participants get after each evaluation, that he's a deep sleeper.

Havel-Lang, 59, who retired two years ago as a technology manager at the state health department, said she signed up for the study in the late 1980s largely because of the money participants receive.

They currently get $150 for an overnight sleep evaluation, $100 for daytime tests of overall health and $35 to fill out questionnaires.

"I was raising two kids by myself and figured I could use a little extra money," said Havel-Lang, of Sun Prairie.

She has learned she kicks her legs in her sleep. "My husband hasn't complained about it; I must have never kicked him," she said.

Whatever their motivation, the participants have made great contributions to sleep science, said Terry Young, lead researcher of the study until last year.

"We could never have had the confidence in our findings and the ability to draw our conclusions without the dedication of the volunteers," Young said.

Apnea not that rare

The study's first major discovery, in 1993, put Wisconsin on the sleep science map.

Sleep apnea was thought to occur in less than 1 percent of adults, but no significant assessments had been done. The development of the first non-surgical treatment for the condition — continuous positive air pressure devices, face masks known as CPAP — made doctors want a better sense of its prevalence.

The Wisconsin study found sleep apnea in 9 percent of men and 4 percent of women.

"That was the landmark that put sleep apnea in the attention of providers, the government and the public," Young said.

An estimated 13 percent of men and 6 percent of women have sleep apnea today, largely because of the obesity epidemic, said Paul Peppard, the study's current leader.

Study participants get overnight sleep evaluations at the hospital every four years. For a while, some came back for daytime nap studies.

Many women underwent sleep studies at home every six months for 10 years to assess the effects of menopause on sleep. Since 2003, most subjects have had tests to measure balance, cardiovascular health and cognitive ability.

With the participants now ages 55 to 85, researchers are exploring the impact of sleep on memory and shifts in sleep patterns after retirement.

The group's younger years revealed a fact many parents can relate to: A parent loses an average of more than 600 hours of sleep per child over the course of a child's life, much of it in the pre-school years.

Minkus, 66, of Madison, who retired as a financial policy adviser six years ago, said he's enjoyed contributing to sleep science.

His evaluations have revealed no sleep apnea or other problems. But they haven't settled a dispute with his wife.

"I'm not sure if I snore," he said. "My wife says I do, but I deny it."