The study, of 218 women treated for early breast cancer, found that almost one-third had "cancer-related fatigue" at the end of treatment. But far fewer -- six percent -- still had the problem a year later.

That suggests for most women with the disease post-treatment fatigue will fade with some time, the researchers report in the Journal of Clinical Oncology.

It's well known that cancer patients often suffer fatigue. And some studies have concluded it's common for that weariness to last for years after treatment ends. Among breast cancer survivors, researchers have found that more than one-third have fatigue two to three years after treatment.

The reasons, though, have not been clear. And the new findings support the idea that some cases of chronic fatigue in earlier studies may have had causes other than the cancer itself.

Past studies have often been "cross-sectional," meaning they studied people at one time-point.

So it's possible to catch some bouts of fatigue that are related to various causes, explained Dr. David Goldstein, who led the new study.

That could mean lingering fatigue from a viral illness, for example, or fatigue related to depression, according to Goldstein, of Prince of Wales Hospital in Randwick, Australia.

"There are cancer survivors with true post-cancer fatigue," Goldstein said in an email. It's just that the number may be "less than previously thought."

That may be true at least with early-stage breast cancer -- and possibly some other early cancers, Goldstein said -- but not necessarily more-advanced cancer.

"The fatigue associated with advanced cancer is a much more complex issue -- tied up with the biology of an active tumor and its many physical effects, as well as the drugs used (for treatment)," Goldstein explained.