Anyone who's entered the fourth decade of life knows the anxiety the approach of a 30th birthday can bring. And those closer to 40 may well be caught in the grips of a full-blown midlife crisis.

It's uncomfortable - but it's perfectly normal, say researchers.In fact, adults go through periods of large-scale renovations of their lives in predictable cycles, says Daniel Levinson, psychology professor at Yale University and author of the 1978 classic "The Seasons of a Man's Life."

Levinson, who is soon to release "The Seasons of a Woman's Life," a companion volume to his earlier work, has studied the lives of research subjects intensively.

"When I look at their life stories, I find each life is unique, but there is this common sequence of periods," he said.

For instance, from 17 to 22, people prepare to begin living as adults. From 22 to 28, they structure a life that's suitable to someone just starting out.

But by age 28, that structure doesn't feel right any more, Levinson says.

"At 28 we enter what I call the `age 30 transition.' We enter a transitional time in which we have to ask, `What kind of life have I made for myself so far as an adult?' and `Is it what I want?' "

From 28 to 33, everyone goes through a period of questioning and change, he said. This may result in a marriage or a divorce, a decision to start a family or a career change. But something big is going to happen, he said.

And, by 33, life settles down again as people begin forming a new life structure - one that will get them to about age 40, when another transition period comes along, Levinson says.

This is followed by another period of stability, then another transition around age 55, when people start preparing for old age.