If you drink a lot of coffee and it has high caffeine levels, the brain doesn't quite work out what is happening

Professor Chris Idzikowski, sleep expert

Lotan Shilo and a team at the Sapir Medical Center at Tel Aviv University, found that six volunteers slept less well after a cup of caffeinated coffee than after drinking the same amount of decaffeinated.

On average, subjects slept 336 minutes per night after drinking caffeinated coffee.

This compares to 415 minutes after decaffeinated coffee.

They took half an hour to drop off to sleep, which is twice as long as usual.

They also moved around in bed twice as much.

Professor Chris Idzikowski, director of the Sleep Assessment Advisory Service in London, said insomniacs attending his clinic are advised to stop drinking coffee from midday onwards.

Caffeine breakdown

He said: "If you drink a lot of coffee and it has high caffeine levels, the brain doesn't quite work out what is happening, so it takes longer to clear it in the body.

"It normally takes two to three hours to metabolise caffeine and reduce it in the system.

"But if you drink a lot of coffee in one go, it takes longer for the blood to break it down."

He said high doses of strong coffee can slow down the metabolism so severely, the caffeine can stay in the body for up to 15 hours.

He said: "This study reinforces the point that if someone is having problems with their sleep that they ought to stop drinking coffee around noon."

In the second phase of the experiment, the researchers woke the volunteers every three hours and asked them to give a urine sample.

Shilo measured concentrations of 6-sulphoxymelatonin, a breakdown product of melatonin.

Stimulant response

The results, published in New Scientist, suggest that melatonin concentrations in caffeine drinkers were half of those found in decaffeinated drinkers.

The researchers suggest that caffeine blocks production of N-acetyltransferase, the enzyme that drives melatonin production.

Professor Ian Hindmarch at the University of Surrey, who studies the effects of drugs on the brain, said people will behave differently to stimulants.

Coffee could affect those with sleep problems

This is due to the law of initial values, which focuses on a person's state of mind at the time they take a drug like caffeine.

He suggests that if coffee is drunk by someone with an aroused or alert brain, it won't have a stimulant effect, but will calm them down.

If someone is more relaxed, then a coffee late at night is more likely to wake them up.

He said: "This is the paradoxical effect of where you start off on the excitement curve.

"I wouldn't go so far as to say no-one should stop drinking coffee at around midday unless they have a sleep initiation problem."