Guarding one's eyes requires tremendous willpower, especially once a person has habituated themselves to looking at everything around them. Many studies have been done on this word called willpower. Sometimes we have more of it, sometimes we have less, and sometimes we have almost none at all.
A study was done in Israel where they looked at many, many cases that came before a parole judge over a long period of time. They wanted to see if there were any metrics that they could find which would predetermine why a person would be granted parole or why they would not. What it turned out was that much of it had nothing to do with the merit or lack of merit of the criminal's case. It had more to do with the time of day that the case came before a judge. Cases that came early in the morning or right after lunch had a much higher probability of achieving parole than those that came before lunch or later in the day. The reason they determined was that granting parole to a criminal—to put somebody back on the street—requires a lot more willpower than just sticking with the status quo and sending them back to prison. Before lunch, when the judge was hungry, or later in the day, when he was exhausted, he was sapped of his willpower and therefore unwilling to grant parole.
What does this have to do with us? You will notice that there are times in the day when you have more willpower to avert your eyes and times when you don't. I noticed this years ago, when I would still take the train or the subway to and from my job. On the way there, I was much, much better than on the way home, when I was exhausted from a day of work. When a person is sapped of willpower, they will succumb to almost anything, whether it's a fattening cheese danish that they know they shouldn't eat, sometimes alcohol, or forbidden sites. It would behoove us to be much more mindful of where our willpower reservoir stands and to create strategies around that. When you know that you're sapped of willpower because you're having a bad day, you're exhausted, or you're just plain hungry, you probably shouldn't go to places where you know you're going to have Nisyonot (tests).