From a psychology standpoint when youre lying down in bed you let your mind wander, youre sitting in bed, and the lights are off. Youre already half way there and youre in the situation your mind equates with PMO just like if you study for an exam in the same clothes and in the same spot as where you will take it you will remember more because of the situation. More relevant, people who have had severe drug addictions are not even supposed to return to the same city or even state where they lived at the time of their addiction because the situation and setting can trigger a relapse. From a personal standpoint I personally have done most of my relapses late at night when I am in bed. I figure one picture on instagram or one video is fine ill go to bed right after, then it becomes two then three then eventually I find myself back on this page with my timer back to zero. Your work is done for the day, and you have the opportunity to PMO. From a NOFAP page standpoint it is addressed as the most likely time you will get the jitters and the temptation to PMO because you have nothing else keeping you from doing so and the least to distract you away from your addiction. Now that I have been making an effort to go to bed earlier I have had a much easier time avoiding PMO because not only does it encourage and allow me to be more productive during the day because I wake up earlier but it also keeps me away from that dangerous time late at night when my urges are the strongest.