Scientists say older consumers should be wary of melatonin ads based on the premise that as we age, the hormone decreases in our system.

Results of a new study fly in the face of this notion, they say. In the study, Dr. Charles Czeisler and colleagues at Harvard Medical School and Brigham and Women's Hospital in Boston studied 34 healthy men and women ranging in age from 65 to 81.

They found their night-time melatonin levels did not differ significantly from those of 98 men in the 18 to 30 age range. The five- year study appears in the American Journal of Medicine.

"In our analysis, we did not find any statistically significant difference in night-time melatonin concentrations between the young and older subjects, although our study does not address whether melatonin levels change after the eighth decade," Czeisler said.

"This means that in most healthy people, concentrations of melatonin in plasma probably do not decline with aging." Melatonin is secreted by the pineal gland, deep within the brain.

The hormone may be a natural sleep inducer. It is produced at high levels during a person's normal sleeping hours and is low during the day. A number of factors, including light and many common medications, can affect melatonin secretion.