Drinking coffee before bed disrupts the body's internal clock, making it harder to get to sleep on time and more challenging to wake up, according to a US study.
The findings, published in the journal Science Translational Medicine, not only explain why evening caffeine drinkers may stay up - and wake up - late, but could also offer travellers a way to time their caffeine use to limit the effects of jet lag.
The study involved five volunteers who were randomly assigned to consume as much caffeine as contained in a double espresso three hours before bed, or who were exposed to bright lights, or given a placebo.
Over 49 days, the volunteers were studied under various conditions, and their saliva was tested for levels of the hormone melatonin, which naturally regulates sleep and waking cycles.
Those who took caffeine in low-light conditions experienced about a "40-minute phase delay of the circadian melatonin rhythm", the study said.
The circadian clocks of those exposed to bright overhead lights three hours before bed bumped back by 85 minutes, and those who took both the caffeine and sat under bright lights were disrupted by 105 minutes.
"This is the first study to show that caffeine, the mostly widely used psychoactive drug in the world, has an influence on the human circadian clock," University of Colorado professor Kenneth Wright said.
"It also provides new and exciting insights into the effects of caffeine on human physiology."
While the findings reinforce the common advice to avoid caffeine before bed, the study also raises the "intriguing" possibility that proper use of caffeine could help reset the body's clock to avoid jet lag.
However, more research is needed to determine how travellers crossing time zones should best use caffeine to stay alert.
"It will be important to monitor for caffeine-induced sleep disruption under such conditions, which could worsen jet lag," the study said.