Q: Do body rhythms exist that can really make a difference in how we feel and function, or is it just a popular myth?

A: The notion that some chartable, intersecting bio-rhythms tend to control our lives -- bringing disaster or success, depending on where we are on the curves -- is mostly nonsense. Solid research doesn't support it at all, yet some people are making quite a lot of money selling useless charts. As astronomer Carl Sagan likes to put it, we're being bamboozled.

On the other hand, there is no doubt, in scientific terms that our bodies are adapted to the 24-hour light-and-dark cycles. These physiological adaptations, or rhythms, are usually called circadian rhythms. This term comes from the Latin "circadies," meaning "about a day."

The things that vary in those periods are sleeping, body temperature, heart rate, kidney function, etc. Body temperature is a very reliable indicator. Your temperature tends to drop one or two degrees during the night, begins to rise in the morning; then reaches an afternoon peak before starting to drop back again.

In addition, any other body functions can be measured, including hormone releases, as they vary over the 24-hour period.

All such patterns can significantly affect people who make sudden changes in their patterns, such as a radical shift in job time, or long-distance air travel. For those who must function at top form, such as athletes, diplomats and business executives, time changes must be watched carefully.

A good rule of thumb is that it takes about a day to adjust for each hour of time change, in terms of the light and dark cycle.

So, if you fly coast to coast, it should take you about three days to adjust fully to the new time or rhythm cycle. You can prepare for such changes if you start living according to the demands of the time zone you'll be entering. Or you can simply adjust to the new time once you've arrived.

Q: You recently wrote that a hernia near an infant's belly button usually corrects itself without surgery. But what about a hernia that develops in a baby's groin area?

A: Unlike hernias near the belly button, infant or childhood hernias in the groin -- or inguinal -- area are more likely to require surgical correction.

The general rule is that children's inguinal hernias need correction through surgery, while umbilical hernias usually do not.

Q: Is there any evidence that ultrasound treatments may cause cancer?

A: There's no evidence so far. But we don't yet have follow-up data covering long periods of time.

As we've seen in the past with X-rays, some drugs and other treatments, there's always the possibility that something may turn up in the future. At present, however, it appears unlikely. Most of the experts think that ultra-sound never will be shown to cause cancer.