Our natural daily 24-hour cycle could be stretched by an extra hour safely and simply by exposure to pulses of bright light, research suggests.
We are governed by an internal 24-hour clock
Experts say it could prove useful for astronauts adapting for long-term missions to Mars - where a day lasts an extra 40 minutes.
The team, from universities in the US and France, tested the light treatment successfully on 12 volunteers.
The study features in the Proceeding of the National Academy of Sciences.
Many species, including humans, have natural "circadian rhythms" set to match the standard length day on Earth.
The contrast between exposure to daylight and night-time darkness is thought to adjust and maintain this clock, which helps makes sure the body is working as effectively as possible at times of day when maximum alertness is required.
Scientists already know that it is possible to interfere with the human circadian "pacemaker" by controlling exposure to light.
The latest research project, shared between Lyon University in France, and Harvard University and Medical School in the US, looked at whether it was possible to "fine-tune" these alterations to achieve a precise result.
Humans do not have precise 24-hour cycles to begin with, and all of the 12 volunteers had cycles ranging from 23.5 to 24.5 hours.
After allowing them to sleep and wake normally for a few days, a new regime was imposed, with artificial "days" produced by a combination of low light and very bright pulses of light near the end of the intended "wakeful" hours.
After 30 days, scientists found that a combination of light brightness and pulses was able to manipulate the circadian rhythm, over time adding approximately one hour to each subject's day.
The researchers said that there were numerous situations in which the ability to do this might be useful.
"Jet-lag, shift work and circadian disorders such as advanced and delayed sleep phase syndromes are all associated, to different extents, with a condition where the circadian system is out of synchrony with the light/dark cycle," the wrote.
Another possible application might be during long space missions - and to allow astronauts to adapt to longer days on Mars.
"In these situations, sleep and circadian disruptions could have serious consequences on the effectiveness, health and safety of astronaut crews," the team told the PNAS journal.
They scientists suggested that the light "treatment" could be administered as Mars astronauts tended crops at the appropriate times of day in a brightly lit greenhouse module.
However, the volunteers in the study were all closeted in a laboratory for more than 60 days in total to avoid unwanted exposure to daylight interfering with the experiment.
Professor Derk-Jan Dijk, from the Surrey Sleep Research Centre, UK, said the study suggested that circadian cycles could be controlled more precisely - but only in controlled environments in which people can be exposed to precise light levels at the right times of day.
He added: "If you have, for example, shift workers on oil rigs who never see the light of day, or people travelling through space, it may be possible to use these methods.
"For this to work, you have to able to avoid light sometimes, which is more difficult in real-life situations, where people are exposed to different levels of light as they go about their normal day-to-day business."