"Morning people", who are more alert early in the day, are more likely to cheat and behave unethically in the night hours, researchers say.
Psychologists found that early-rising "larks" and late-night "owls" had different levels of honesty depending on the time of day.
The study found a link between ethical choices and such internal clocks.
Sunita Sah, research fellow at Harvard University in the US, said this had "implications for workplaces".
The research examined the behaviour of almost 200 people - with the subjects taking part in problem-solving tests and games without realising that it was their honesty that was being measured.
The study, The Morality of Larks and Owls, examined the relationship between ethical decision making and people's "chronotype" - which is when individuals are most likely to want to be asleep or when they have more energy.
It found a significant link between people being more likely to be honest when it fitted in with their chronotype, This meant that the early-rising "larks" were more ethical in the morning - and the "owls" were more likely to be honest at night.
The study used financial rewards of up to $10 for completing tests against time - and contests with the prospect of bigger prizes. But researchers were really monitoring how people self-reported their results.
The level of dishonesty was found to be heightened when people were outside of their preferred time of day. The study reports: "In the morning, evening people are more unethical than morning people."
It found that "ethical behaviour arises when people 'match' their situations".
The research was carried out by academics at US universities - Johns Hopkins University and the University of Washington, and Prof Sah is an assistant professor of business ethics at Georgetown University as well as a research fellow at Harvard.
Prof Sah, assistant professor of ethics at Georgetown University's McDonough School of Business, says that the findings have major implications for workplaces relying on ethical decisions and honesty - particularly where there are shift patterns.
It raises questions about working hours and the structure of the working day, she says, if people's decision making is affected by their chronotype.
Making an ethical choice - such as sticking to the rules of a test - seems to change with people's internal body clocks and different times of the day, suggests this study.
But the findings also challenge suggestions that night owls are more likely to be badly behaved.
"They cast doubt on the stereotype that evening people are somehow dissolute," the report concludes.