"Caffeine is very safe; it's used in newborns to increase arousal," said John Herman, professor in sleep medicine at University of Texas Southwestern Medical Center in Dallas.
"But nothing should be packaged that could appeal to children," Herman said. "It should specify dosage and instructions on how to use it and what is the maximum. A high dose of anything -- sugar or salt -- becomes harmful.
"If it's red-colored and it's sweet, kids might take three, four or five of them and go into an anxiety attack and palpitations," he said. "Kids get anxious when they take caffeine and it could put them over the top."
Even one cup of coffee in the morning can affect sleep, which can be more fragmented and lead to early morning awakening, Herman said.
Caffeine intake is growing among children at younger ages, but there are no long-term studies on its effect on child development, according to James W. Wyatt, a clinical psychologist at the Clinic for Sleep Disorders at Rush University.
Teenagers need eight to nine hours of sleep a night so they can "learn optimally," he said. "Everything from attention, concentration, memory and social skills ... they all rely on an alert brain."
Repeated sleep loss can compromise the immune system.
Rush University sleep expert Cartwright said high doses of caffeine can even cause dangerous sleep aberrations.
"When you get over their threshold of response, you get rebound sleep that is like a narcolepsy attack and it's very scary for them and for anyone trying to wake them out of it," she said.
In some severe cases, Cartwright said such fatigue can result in a dangerous half-awake, half-asleep state where the brain plays a tug-of-war with itself. A 2011 study published in the journal Nature described a similar state in rats.
"You get up and walk, but the brain is not fully awake," she said. "There are enough cases of sleep walkers in the literature who took an enormous amount of caffeine during the day and into the night."
Cartwright defended a court case in 2008 where a man walked into the bedroom of a neighbor's sleeping teenager and boyfriend while the accused was asleep.
"He was caffeine loaded," Cartwright said. "But we don't know who's vulnerable and who's not."