People who are more alert early in the day are more likely to cheat and behave unethically in the night hours, researchers in the United States say.
Psychologists have found early rising "larks" and late night "owls" have different levels of honesty depending on the time of day, the BBC reports.
The study found a link between ethical choices and internal clocks.
Harvard University research fellow Sunita Sah said this had implications for workplaces.
The study 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 the early-rising "larks" were more ethical in the morning - and the "owls" were more likely to be honest at night.
The research examined the behaviour of almost 200 people - with the subjects taking part in problem-solving tests and games without realising it was their honesty which was being measured.