Most of us strive to do the right thing when faced with difficult decisions. A new study suggests that our moral compass is more reliable when we face those decisions in the morning rather than later in the day.

In a series of studies at Harvard University and at the University of Utah, 327 men and women participated in tasks designed to measure cheating or lying behavior either in the morning or in the afternoon. For instance, in one study the subjects attempted to solve math problems, some of which were impossible, knowing they would be paid five cents for every solved problem. They reported their own scores, giving them an opportunity to lie and thus receive more money. The people who participated in the afternoon sessions in all the experiments were more likely to cheat than those who took part in the morning sessions.

Ethical decisions often require self-control, which past research has found to be dependent on the body's energy stores, much like a muscle: if it is heavily taxed, it eventually becomes exhausted. This study suggests that even the regular activities of daily life can deplete these resources. It also hints that sleep is crucial for rebuilding moral muscle; indeed, previous research shows that sleep deprivation hampers ethical decision making. So if you are faced with an ethical dilemma, you may want to save your pondering for the morning after a good night's sleep.