Caffeine affects boys more than girls, but only after they hit puberty, according to a new study published in the latest edition of the journal Pediatrics. Jennifer Temple, lead study author and an associate professor at the University at Buffalo School of Public Health and Health Professions, first noticed a trend a few years ago while researching children's diets. Temple, who has studied caffeine for about eight years, was surprised by the amount of soda or coffee the children in her studies were drinking. When she looked at the medical literature, there was little that spoke about the impact caffeine might have on growing bodies. So that's what her team decided to study. "We didn't go into this research with a hypothesis about sex differences," Temple said, "but that's what kept coming up in our research." About 73% of children have some kind of caffeine daily, the American Academy of Pediatrics says. AAP would prefer that children not consume caffeine. The Food and Drug Administration does not set a standard for what is considered a safe amount for children. For adults, a safe daily level, the FDA says, is the equivalent of what is found in about four or five cups of coffee. How dangerous is sleep deprivation? Caffeine has no nutritional value. While it is a psychoactive stimulant that can improve alertness and moods, it is also habit forming. In large doses it can cause nervousness, tension and sleeplessness. It can also increase blood pressure and lower a person's heart rate, no matter what their age. Ninety-five children participated in Temple's study. Scientists gave them three different drinks: orange juice, a lemon-lime flavored soda, and a kind of lemonade with either a placebo added or caffeine. Neither the children nor their parents knew which of the drinks had caffeine. The scientists then tested the children's blood pressure and heart rate on six occasions. They found no gender difference in the way caffeine affected younger kids -- children age 8 to 9. But when they looked at the older children's results, they found boys between 15 and 17 reacted more immediately to caffeine than girls did. Teen boys' heart rates decreased more and their systolic blood pressure went up higher than girls'. Similar sex differences have been found in adults in past studies. The difference maker, scientists think, is puberty. They aren't sure, however, if it is something physiological -- like the amount of steroid hormones an adolescent makes compared to a younger child -- or if it is due to other factors. The scientists also saw a difference in the way caffeine affected girls depending on where they were in their menstrual cycle. Decreases in heart rate were greater if a girl was in the mid-follicular phase of her period (near the beginning of the cycle), compared to when she was in the mid-luteal stage (toward the end of the cycle right before menstruation).