Scientists believe they have figured out why it takes us so long to adapt when we travel to new time zones.
Researchers at Oxford University say they have found the "molecular brakes" that prevent light resetting the body clock when we fly - causing jet lag.
Experiments, reported in the journal Cell, showed "uncoupling" these brakes in mice allowed them to rapidly adapt.
Researchers hope the discovery will help find new drugs for jet lag and mental health treatments.
The body clock keeps us in tune with the pattern of day and night. It means we sleep at night, but also affects hunger, mood and blood pressure.
Light acts like a reset button to keep the clock to time, but when we fly around the world it takes time for our body clocks to adjust. The resulting fatigue, which can last for days, is known as jet lag
The research team, funded by The Wellcome Trust, was trying to figure out why people do not instantly adapt. They looked in mice as all mammals have the same core body clock mechanisms.
They focused on the "master clock" in a part of the brain, which keeps the rest of the body in sync, called the suprachiasmatic nuclei
They were looking for sections of DNA that changed their activity levels in response to light.
They found a huge numbers of genes were activated, but then a protein called SIK1 went round turning them all off again. It was acting as a brake by limiting the effect of light.
Experiments to reduce the function of SIK1 meant the mice could rapidly adjust their body clock when it was shifted six hours - the equivalent of a flight from the UK to India.
Prof Russell Foster told the BBC: "We reduced levels by 50-60%, which is big enough to get a very, very big effect. What we saw was the mice would actually advance their clock six hours within a day [rather than taking six days for untreated mice].
"We've know there's been a brake on the clock for some time, but we had absolutely no idea what it is, this provides a molecular basis for jet lag and as a result new targets for potentially developing new drugs."
He said some mental health disorders including schizophrenia were linked to an out-of-tune body clock, so these findings may open up new areas for research.
The brakes are likely to be in place to prevent the body clock from becoming erratic and being reset by artificial or moon light.
Dr Akhilesh Reddy, a specialist in the body clock, at the University of Cambridge, was very confident that treatments would follow as "it is a very drugable target and I would suspect there are lots of potential drugs already developed".
He told the BBC: "We have known a lot about the basis of jet lag and why it occurs.
"This shows how you can get into the brain and manipulated the clock, which is why this study is important.
"We have drugs which can make the clock shorter or longer, what we need is to shift it to a new time zone and that is what they have done."