Timed flashes of light during sleep may be the best way to combat jet lag, a study has found.
Researchers found that exposing volunteers to short "camera flashes" of light as they slept reset their body clocks.
Using the flash therapy the night before making a long trip could help a traveller quickly adjust to a new time zone, the findings showed.
Jet lag, which occurs when the body is out of sync with a destination's sleeping and waking hours, can cause fatigue, poor concentration and performance, a general feeling of malaise, and stomach upsets.
Light therapy treatments for sleep disturbance, which involve sitting in front of bright lights during the day for hours at a time, were already known to alleviate symptoms of jet lag.
But a new study of volunteers has shown that exposure to brief bursts of light while sleeping is far more effective.
Lead scientist Dr Jamie Zeitzer, from Stanford University Medical Centre in the US, said: "This could be a new way of adjusting much more quickly to time changes than other methods in use today."
Even when the eyes are closed during sleep, light triggers signals from the retina to the "circadian system" in the brain that alters the biological clock.
For the study, Dr Zeitzer's team recruited 39 volunteers aged 19 to 36 who were given a routine sleep-wake cycle for two weeks, going to bed and waking up at the same time every day.
Then the participants were exposed either to various frequencies of flashing light or continuous light for an hour as they slept.
The researchers found that two millisecond long bursts of light - similar to a camera flash - set off at 10-second intervals delayed the onset of sleepiness the next day by nearly two hours.
For volunteers exposed to continuous light, the delay was only 36 minutes.
Dr Zeitzer explained how flashing light therapy could help someone flying more than 2,000 miles from California to New York. The US West Coast is three hours behind the East Coast.
He said: "If you are flying to New York tomorrow, tonight you use the light therapy. If you normally wake up at 8am, you set the flashing light to go off at 5am. When you get to New York, your biological system is already in the process of shifting to East Coast time."
Dr Zeitzer added: "We have found that most people can sleep through the flashing light just fine."
The flashing light treatment could also help other groups who suffer body clock disturbance, from night shift workers to truck drivers, say the researchers whose findings are reported in the Journal of Clinical Investigation.
Flashing light is said to work better than continuous light partly because the gaps of darkness between flashes allow pigments in the eye that respond to light to be re-activated.
The retinal cells that transmit the light information to the brain also continue to fire for several minutes after the stimulus, said Dr Zeitzer.