A new study found caffeine had no impact on sleep. Photo / Getty Images
A new study found caffeine had no impact on sleep. Photo / Getty Images
Caffeine is known to be a powerful stimulant which perks up drinkers, increases alertness and promotes wakefulness.
Yet a surprising new study has found that drinking tea or coffee within four hours of bedtime does not actually impact sleep at all.
US researchers from Florida Atlantic University and Harvard MedicalSchool monitored 785 people for a total of 5164 days and nights, recording how much caffeine, alcohol and nicotine they consumed.
They then compared the information to sleep diaries and wrist sensors which recorded sleep duration, sleep efficiency and how quickly people woke up after drifting off.
Around 40 per cent of the group admitted to drinking caffeine on at least one night of the study. Yet although nicotine and alcohol did disrupt sleep - with a night-time cigarette knocking 42 minutes off total sleep for people suffering from insomnia - caffeine had no effect.
The researchers say the findings were unexpected because physiologically, caffeine promotes wakefulness by blocking sleep-promoting chemicals in the brain.
Writing in the journal SLEEP, Dr Christine Spadola, Assistant Professor at Florida Atlantic University, said: "Considering the public health importance of obtaining quality sleep and the widespread use of alcohol, nicotine, and caffeine, relatively few studies have thoroughly investigated the association between evening substance use and sleep parameters.
"This study represents one of the largest longitudinal examinations of the associations of evening use of alcohol, caffeine, and nicotine with objectively measured sleep outcomes.
"A night with use of nicotine and/or alcohol within four hours of bedtime demonstrated worse sleep continuity than a night without these substances. We did not observe an association between ingestion of caffeine within four hours of bedtime with any of the sleep parameters.
"These findings support the importance of sleep health recommendations which promote the restriction of evening nicotine and alcohol use."
A recent report by the Sleep Council found 70 per cent of adults in Britain now get less than seven hours of sleep a night and more than one-quarter experience poor sleep on a regular basis.
The National Health Service (NHS) in England currently recommends that people should cut down on tea, coffee, energy drinks or cola in the evening because caffeine interferes with the process of falling asleep and prevents deep sleep. The health service recommends warm milky drinks or herbal teas instead.
The advice is based on previous research which showed that caffeine use close to bedtime appears to increase the time it takes to fall asleep and makes people wake up sooner.
The NHS recommends warm milky drinks or herbal teas in the evenings instead. Photo / Getty Images
But the researchers say that most studies have involved small numbers of participants, in unnatural laboratory settings, where they were given caffeine tablets to mimic the effects of nighttime drinks.
In contrast, the people in the new study went about their normal lives.
The results show that a night with evening nicotine use - compared to a night without - also led people to wake up six minutes earlier after nodding off.
However, the researchers said that they were unable to say how much caffeine each participant had or if any were particularly sensitive.
