When I have a morning lecture, I sometimes walk in with a cup of coffee. And so did a lot of my students.

But over the years, the beverage of choice has evolved from coffee to soft drinks and now to energy drinks.

I've noticed that. They're popular with young people because these drinks claim to give you extra pep.

They contain ingredients such as tuarine, guarana (gwa-ra-NAH), ginseng, B–vitamins and most of all, lots of caffeine.

If people down energy drinks to improve their mental and physical performance, there's little evidence they work. Nearly all the ingredients are stimulants.

Guarana seeds have up to three times as much caffeine as coffee. Ginseng, which claims to boost the immune system, is based on paltry scientific evidence.

There's also sugar and of course, caffeine.

Some brands contain up to 300 milligrams of caffeine in an 8 ounce serving. But some people drink more than one can at a time. If you're a healthy adult, research shows that's generally fine.

But women of reproductive age are advised to limit caffeine to no more than 300 milligrams a day. And adolescents shouldn't have more than 100 milligrams a day.

Yet, young people like energy drinks, which can cause children to be hyperactive, fidgety or even enraged.

Caffeine blocks a brain chemical called adenosine, which is involved in sleep. With excessive caffeine, neurons start firing, putting the body in emergency mode. Then the pituitary gland releases adrenaline, which raises the heartbeat and dilates the eyes.

These physical responses make you feel as though you have more energy when you don't. So, you won't run any faster or get more right answers on a test.

In general, energy drinks are safe for adults – in moderation. Remember, caffeine is addictive and a diuretic. So, don't risk dehydration by drinking these while exercising.