A large dose of caffeine may be the way many of us start the day, but researchers say little and often would do more to help us stay awake.
A crucial part of the morning routine for many
Harvard University researchers say a morning coffee boosts caffeine levels, but these fall away during the day.
They say frequent low doses of caffeine would give people such as shift workers who need to stay awake more of boost.
Writing in the journal Sleep, they say caffeine works by interfering with one of the systems which governs sleep.
When we sleep, and for how long, is regulated by both the circadian system and the homeostatic system.
The circadian system is tuned in to the difference between night and day, and promotes sleep rhythmically, with an internal clock releasing melatonin and other hormones in a cyclical fashion.
But the homeostatic system is demand driven - it tells the body it needs more sleep the longer someone has been awake.
Researchers from Rush University Medical Center, Brigham and Women's Hospital and Harvard Medical School studied 16 men who lived in private suites for 29 days, without any kind of time cues.
The men were scheduled to live on a 43-hour day - remaining awake for periods of around 28 hours, so that they had to stay awake for the same kind of time as doctors or people in the military have to.
The extended day was also designed to disrupt the subjects' circadian system while maximising the effects of the homeostatic push for sleep.
The men were given either caffeine pills, roughly the equivalent of two ounces of coffee, or dummy pills. They took the pills upon waking and then once every hour.
Neither the men nor the researchers knew who was taking what.
Following the study, it was found the men who took the caffeine pills had fewer microsleeps, where someone accidentally falls asleep very briefly.
Electroencephalogram (EEG) tests, which detects electrical activity in the brain, showed those who took the dummy pills were unintentionally asleep 1.57% of the time during which they should have been awake, compared to 0.32% for those taking the caffeine pills.
Those taking the caffeine were also found to perform better in cognitive tests.
However, even though they were better able to stay awake, they reported feeling sleepier than those taking dummy pills, suggesting that while caffeine can help you to stay awake, it does not replace the restorative effects gained through sleep.
'No substitute for sleep'
The researchers suggest caffeine blocks the action of receptors for adenosine, a key chemical messenger in the homeostatic drive for sleep.
Dr James Wyatt, a sleep researcher at Rush University Medical Center, the lead author on the study, said: "I hate to say it, but most of the population is using caffeine the wrong way by drinking a few mugs of coffee or tea in the morning.
"This means that caffeine levels in the brain will be falling as the day goes on.
"Unfortunately, the physiological process they need to counteract is not a major player until the latter half of the day."
Dr Wyatt added: "While there is no perfect substitute for sleep, our results point the way toward a much better method for using caffeine in order to maintain optimal vigilance and attention, particularly when someone has to remain awake longer than the traditional 16-hour wake episode."
Neil Stanley, chairman of the British Sleep Society, told BBC News Online: "You probably buy yourself an extra half hour of wakefulness with a low dose of caffeine in a cup of tea or coffee.
"Caffeine does affect the adenosine receptors, but we can't say for sure that's how it affects the body's sleep desires.
"But, if you are feeling sleepy during the day, a 20-minute nap beats three cups of coffee any day. That's what your body needs."
Previous research has suggested caffeine can raise blood pressure, and that drinking more than eight cups of coffee a day in late pregnancy doubled the risk of a -baby being stillborn.