It seems that things like being thirsty, hungry, having to go to toilet, being sleepy, hearing noises from the outside or from another room, can be easily ignored and almost unnoticed (at least while they are at low to moderate intensity) while we're doing fun things, such as surfing the Internet. But the very moment we start to work, or study, we suddenly notice we need some water, or food, or our brain doesn't work properly and needs some sleep, or we need to go to toilet, or noise from another room starts bothering us, even though we didn't notice it while we were doing the fun things. Seems like our tolerance for any discomfort is much lower when we need to work or study. Why is that so?