Having a high sugar drink to boost energy can actually make people more sleepy, a study suggests.
Only sleep will really help overcome tiredness
Loughborough University researchers say the sugar rush gives a short respite.
But after an hour, people who had such drinks had slower reactions and more lapses in concentration than those who had a no-sugar, no caffeine drink.
Sleep experts say energy drinks help athletes, but sleepy drivers or others needing a boost should have a small amount of caffeine - and a short nap.
In the Loughborough study, published in the Human Psychopharmacology: Clinical and Experimental journal, 10 adults were studied to see what effect different drinks had on their wakefulness.
They had all volunteered to restrict their sleep to five hours on the day before participating in the trial.
An hour after eating a light lunch they were given either an energy drink containing 42g of sugar and 30 milligrams of caffeine, or an identically tasting zero-sugar drink.
They then performed a monotonous 90-minute test during the afternoon "dip" to assess their sleepiness and ability to concentrate.
The researchers found that for the first 30 minutes there was no difference in the reaction times or error rates.
But 50 minutes after consuming the drinks, the performance of those who had had the energy drink started to slip, and they became significantly sleepier.
'Have a cat nap'
Professor Jim Horne, who runs the Sleep Research Centre at the University of Loughborough. said: "A 'sugar rush' is not very effective in combating sleepiness - so avoid soft drinks that contain lots of sugar but little or no caffeine.
"A much better way to combat sleepiness is to have a drink that contains more useful amounts of caffeine and combine this with a short nap."
Professor Horne said people with low blood sugar would feel better after having an energy drink.
But he added: "Whilst there is good evidence that sugar intake can boost physical energy, there is little support for it having any benefits for a sleepy brain."
Drivers who are feeling sleepy are advised to take precisely those measures to help them reach their destination.
But Dr Neil Stanley of the British Sleep Society said the only thing your body needs when you start to feel tired is sleep.
"We live in a tired society - which is why these drinks exist."
But he added: "Energy drinks boost blood sugar. They are good for athletes. But if you're sitting still, you're not doing anything with that energy."
Dr Stanley said drivers and others who were feeling tired should turn to caffeine if they could not get the sleep their body wanted.
"As the Highway Code recommends, you should have a couple of caffeine drinks and a cat nap."
"But when you come down, you'll be back to where you were."