The question of why good people do bad things has fascinated psychologists for decades. Stanley Milgram famously identified deference to authority as a factor pushing people toward unethical behavior, but it seems that something even simpler could be in the mix: fatigue.

A recent study published in Psychological Science found that people are more inclined to cheat on a task at different times of the day, depending on their individual body clocks, or “chronotypes.” Chronotypes affect people’s natural peaks and troughs of physical and cognitive functions throughout the day, making “larks” more alert first thing in the morning and “owls” more wakeful late at night. The new evidence suggests that morning people are more likely to cheat at night, while evening people are more likely to cheat in the morning.

Morality in the morning

Building on research suggesting that people are more dishonest when they are tired, Brian Gunia, Christopher Barnes, and Sunita Sah assessed the chronotypes of participants, classifying them as either morning, intermediate, or evening people. Participants attended morning test sessions that required completing a puzzle task and were paid $0.50 for each puzzle they claimed to have solved correctly. If a participant failed to solve a puzzle but reported having done so, this was counted as a cheat.

The results showed that evening people cheated more often than intermediate people, who in turn cheated more often than morning people. The number of people who cheated within each group was also affected: a lower percentage of morning people cheated than intermediate people, and fewer intermediate people cheated than evening people.

However, it could be that evening people are more inclined to cheat at any time of day, not just in the morning. To establish whether this could have affected the results, the researchers performed similar tests in the evenings. They found that evening people were indeed inclined to cheat less in the evening than in the morning, showing the opposite pattern to morning people.

The study appears to contradict previous research, also published in Psychological Science, showing a “morning morality effect” that saw people cheating more on a task in the evening than in the morning. Maryam Kouchaki and Isaac Smith, the authors of the paper on morning morality, suggested that the simple drain of living through the day, such as making decisions and expending physical energy, can deplete resources needed for self-control.

Dozens of empirical studies show that each act of self-control draws on the same finite well of self-discipline and makes each subsequent act of self-control more difficult. Anyone trying to change their eating habits will attest to the fact that refusing the third offer of junk food is much more difficult than refusing the first.

Morality is one of the behaviors that seems to rely on self-control. After completing a task that requires self-control, participants score lower in a subsequent task requiring moral awareness. In a series of experiments designed to test the morning morality effect, this influence of lowered self-control was evident: participants cheated significantly more on a task conducted in the afternoon than on the same task conducted in the morning.

Night owl ethics

However, the morning morality effect “overlooks the fact that 40 percent of people experience an increase in energy as the day wears on” write Gunia, Barnes, and Sah in this follow-up investigation of chronotypes. While morning people may be firing on all cylinders at 8am, evening people might feel more alert—and have more self-control—at 10pm.

The new results can actually help us identify the biological processes that influence morality. Sleep is governed by two separate but concurrent processes, the authors explain. One is simply how many hours a person has been awake: the longer the period of wakefulness, the more inclined to sleep the person becomes. These ‘homeostatic’ processes cause a build up of sleep regulating substances in the body, increasing fatigue throughout the day.

Meanwhile, the influence commonly known as circadian rhythm creates night-day cycles in the inclination to fall asleep. Circadian and homeostatic processes work together to make morning people feel alert right when they wake up and sleepy in the evenings, whereas for evening people, the two processes conflict in both the morning and the evening. For shift workers, the two processes can conflict completely.

This means that, while morning people will always experience the morning morality effect, evening people can tell us whether homeostatic or circadian processes have a larger effect on morality. If evening people experienced the morning morality effect alongside their peers, that would mean that homeostatic processes are more powerful for self-control. Because the results showed that evening people are more ethical at night, this suggests that circadian processes dominate homeostatic processes.

This raises the question of why the initial research on the morning morality effect found that, overall, people were more moral in the morning. Unless the participants in these studies were somehow mostly morning people, the results of the two papers are contradictory.

Smith and Kouchaki suggest that the timing of the sessions was important: where the morning morality studies had morning sessions between 8am and 11am and afternoon sessions between 3pm and 6pm, the chronotype studies had sessions from 7am to 8:30am and again from midnight to 1:30am. This could mean that evening people experience the morning morality effect over the course of a normal workday but have a circadian boost later on in the day that counteracts the normal effect. More research will be needed to work out whether the effects are stronger or weaker at various points throughout the day.

Understanding these kinds of effects could have important implications for fields ranging from education and business to medicine. Teenagers have the latest chronotypes of any age group, yet often write tests and exams early in the day. The case could be made that businesses should increase vigilance for employee and customer dishonesty later in the day. And perhaps most importantly, depletion of self-control seems to impact medical professionals’ compliance with guidelines such as hand hygiene. Understanding the effects of fatigue, circadian rhythms, and shift work on medical risk factors like this could help us to reduce patient risk.

Psychological Science, 2014. DOI: 10.1177/0956797614541989 (About DOIs).