We all know that our willpower can be drained as the day progresses, but it turns out that as we become more tired, we lose all kinds of faculties. Scientific American points out that time time of day can an effect on whether or not we tell the truth.
Research published in Psychological Science shows that as the day goes on, normally truthful people can become unethical:
Scientists showed volunteers patterns of dots on a computer and asked them to tell which side of the screen contained more dots, the right or left. Here's the kicker: the researchers gave the subjects a higher reward if they selected the right side, even if it was incorrect. And with this incentive, subjects were more likely to cheat in the afternoon than in the morning.
The idea here is that as the day goes on, we get tired, and that might have a correlation with how willing we are to tell the truth. If you've ever had one of those exhausted, white lie moments, this might explain why.
We Are More Likely to Lie in the Afternoon [Scientific American]