Researchers at the University of Georgia find association between fitness levels and sleep complaints.

Rodney Dishman, a professor of kinesiology in the College of Education, said that as people grow older, they risk losing their cardiorespiratory fitness. Normally, adults around the age of 45 years, will start losing fitness if they aren't exercising regularly. The more time a person spends on a treadmill, the greater cardiorespiratory fitness is.

"When participants lost a minute on their treadmill time, they experienced a greater risk of sleeping problems," Dishman said in a statement. "Our findings give an incentive for adults to maintain fitness and continue exercising."

The study, from the Aerobics Center Longitudinal Study, followed a group of more than 8,000 men and women, between the ages of 20 and 85, from 1971 to 2006. The participant's cardiorespiratory fitness was measured over four clinic visits during a period of two years.

The researchers found that participants did not complain about sleeping problems during their first visit. However, over time, a significant number of participants lost their cardiorespiratory fitness and their treadmill endurance decreased. During their second or third visit, they were no longer able to exercise like they could during their first visit.

The study also found that each minute decrease in treadmill endurance between the ages of 51 and 56, led to rise in sleep complaints by 1.7 percent for men and 1.3 percent for women. Plus each minute of treadmill decline, sleep complaints increased by 8 percent during second or third visits.

"Fitness is much harder to sustain if you don't exercise consistently," Dishman said. "It says that adults should get 150 minutes of moderate physical activity or 75 minutes of vigorous activity per week. Examples include brisk walking for moderate activity and jogging, biking and swimming for vigorous activity. Staying active won't cure sleep complaints, but it will reduce the odds of them," Dishman said. "The more active you stay, the better off you'll be."

The study on "Decline in Cardiorespiratory Fitness and Odds of Incident Sleep Complaints" is published in the journal Medicine & Science in Sports & Exercise.