A new survey has found that more than a third of British teenagers are high on caffeine tablets and energy drinks when preparing for exams.
The study led by School Food Trust showed that students revising for exams skipped meals, did less exercise, and ate more junk food.
They often underestimated the importance of healthy living.
Of the 500 students questioned during the survey, 48 per cent thought that eating properly was important to help them revise
And less than one in five felt that exercise was important in their study regime.
Almost 79 per cent agreed that they were likely to snack more and eat less healthily when studying or revising.
In addition, 42 per cent said that they had skipped meals to make time to revise, and nine in ten regularly felt tired because of their schoolwork.
Consequently, only half of teenagers could study only for 30 minutes before losing concentration.
Nearly 26 per cent admitted to using energy drinks, while 11 per cent admitted to using performance enhancers like caffeine tablets.
The study also showed that chocolate was the revision food of choice, chosen by more than 42 per cent of those questioned, followed by 33 per cent fizzy drinks and 31 per cent biscuits.
"It's often said you get out what you put in - our research shows that children are able to perform better in class when they have had a healthy school lunch rather than junk food," the Telegraph quoted Prue Leith, who is chair of the Trust as saying.
"Children aren't stupid and they know that healthy food is better for them, and that a healthy breakfast and a balanced school lunch will give them more energy for their studies and help them concentrate more.
"Making that choice, and sticking with it, especially come exam time, could be the difference between success and failure," she added.