Three recent studies show why exercise -- even when begun late in life -- offers a payoff for older adults. Here's what researchers have learned:

Sleep complaints. A Stanford University School of Medicine study found that sedentary older adults who walked or did aerobics four times a week improved their sleep after a 16-week program. Exercisers slept about an hour longer each night and cut sleep onset time in half.

Knee arthritis. Eight months of walking and strength training helps, not hurts, older people with disabling knee arthritis, say researchers at Wake Forest University. Exercisers reported less disability and pain. They outwalked nonexercising control groups and took less time to climb and descend stairs.

Body fat. A study at Washington University says that during exercise, inactive seniors can get their bodies to burn fat at a rate similar to younger people if they train. Elderly people burn fat 25 to 30 percent slower than younger people. When fat is slow to burn, carbohydrate stores are depleted faster, causing early fatigue. As a result, older people can't exercise as long as younger people.