My scrotum is stretched or loose during the night when I sleep but during day it's tight and round shaped. I am worried about this change. Why is it so? Is this is normal? Is there any relation with masturbation and this kind of change?
Your scrotum, the bag that contains your testicles, performs several important jobs. Its first job is to keep your testicles at just the right temperature so that they can manufacture sperm. This temperature is about 1 to 4 degrees cooler than the rest of your body. This is why your testicles are outside the bulk of your body.
When you sleep, your body temperature rises a few degrees. If you have ever held a baby, you learn this real quick. They get very warm when they are sleeping, but then, so do you. In order to keep your testicles at the right temperature, the scrotum loosens so that more surface area is exposed and the scrotum along with the testicles can cool off. This is why you notice your scrotum being more loose at night.
During the day, you are cooler so the scrotum doesn't hang down as much. You are also much more active and as you walk or run, the scrotum draws up a bit to keep your legs from knocking your testicles around (the scrotum's second job). If you get really hot, such as after an intense game of soccer, and you stop to cool off, your scrotum is also loosening trying to get your testicles cooler.
As you get a bit older, you will be become more muscular. The added muscles will cause your body to produce more heat, even at rest. Thus, as you get closer to adulthood, your scrotum will hang more loosely. However, it will still draw up when you are active or when you get cold.
Masturbation does not change the shape of the scrotum, except on a temporary basis. The activity and sexual tension that comes from masturbating will cause the scrotum to draw up for a short period of time.