That annoying tendency of teen-agers to stay up later may be a good sign they are eating right and their brains have reached a significant stage in development, according to Mari Golub, a behavioral neurobiologist and associate adjunct professor at the University of California, Davis. Golub believes this because of her recent studies of zinc deficiencies in adolescent monkeys, to be presented at the Society for Neuroscience's annual meeting in San Diego on Tuesday, Nov. 14. Her research has revealed a link between zinc deficiency and slower skeletal and brain maturation and slower growth in adolescent monkeys. The shift in sleep habits could be a normal behavioral change reflecting final brain maturation that occurs in adolescence. Monkeys provide a valuable behavioral model for studies of adolescence. According to Golub, they are "the only animals known to go through an adolescent stage like humans, thus providing an opportunity to look into the problems of adolescent neurobiology." Later bedtimes are one of the most dramatic behavioral changes in adolescence. The shift in sleep patterns usually begins in the early teens. Golub's study revealed that the shift occurred in monkeys in the absence of social or physical environmental changes, suggesting the change may not result from emotional or social changes typically associated with adolescence. Golub looked specifically at the role of adequate dietary zinc levels and the changes in bedtimes as a behavioral indicator of brain development. Zinc, an essential mineral to many functions of the body from cell replication to carbohydrate metabolism, is critical during periods of rapid growth as during infancy and adolescence. She chose eight female rhesus monkeys very close in age. Four were fed low-zinc diets, and four were fed the identical diet except for the addition of an adequate amount of zinc. Backpacks containing small monitors designed for hyperactive children were worn by each monkey to record all body movements in 24-hour periods, enabling the researchers to track the active and rest phases of each animal. The monkeys were studied as they aged from 18 to 45 months, about the equivalent of aging from 10 to 16 years for human females. All lived indoors in identical temperature-controlled conditions with lights turned on and off at exactly the same time every day. "The results were startling," says Golub, who found the monkeys fed the normal diet began to have later bedtimes. The sleep patterns of the zinc-deprived monkeys remained unchanged, their growth spurt was attenuated and skeletal maturation slowed. Golub suggests that brain development may have been affected and proposes that perhaps malnourished teen-agers may have other behavioral problems as a result. "Zinc is a covert deficiency. The child could be getting plenty of calories but not enough zinc." The Recommended Daily Allowance (RDA) of zinc for adolescent girls is 12 mg and for boys is 15 mg. Zinc-rich foods are often unavailable to impoverished individuals, and a diet that relies mostly on processed foods and little meat can be lacking in sufficient quantities of zinc. However, if servings of whole grains are a regular dietary component, the correct nutrient amounts are naturally present. A previous study by Golub revealed the importance of dietary zinc during pregnancy. She plans to continue her work with her adolescent animal model with a new study looking at brain and sexual maturation relationships. This research is part of a series of ongoing studies, supported by the National Institutes of Health since 1980, that involves a team of scientists trained in nutrition, psychology, immunology, pediatrics and primatology. The effects of low dietary zinc were researched at the California Regional Primate Research Center at UC Davis.