Not to give anyone an excuse for bad behavior or poor choices, but researchers are saying that "cognitive tiredness" later in the day can play a role in the decision to give in to temptation.
A number of studies seem to reveal similar findings. In one study, folks were far more likely to cheat on a task in the afternoon than in the morning. In another study, they were more likely to cheat after doing other tasks (like memorizing numbers) that left their brains somewhat fatigued.
Other researchers don't agree. They say the studies that support what some call "morning morality" don't take into consideration the difference between "morning" and "night" people. Their point is that you can't really assume someone is more cognitively tired in the afternoon because that person may actually be operating at their peak at that time of day.