Strong coffee can wreak havoc on sleep patterns
Coffee is not just a stimulant, it interrupts the flow of melatonin, the brain hormone that sends people to sleep.
Melatonin levels start to rise about two hours before bed time and peak between 0200 and 0400.
However, researchers in Israel have found that caffeinated coffee halves the body's levels of the sleep hormone.
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.
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."