Mishnah Berurah — משנה ברורה

To also recite "Who created..." - The reason is because in the morning we are like a new creation and we need to give thanks to Him, and the blessings of "Elokei Neshama" ("My Lord, the soul [you have given me]...") and "Asher Yatzar" ("Who created [man]...") were established in regards to [Hashem's] forming our bodies. There are other reasons given - see further in the later rabbinic authorities. The Pri Megadim wrote that based on the reason of [us being] a new creation, if one was up all night, in which case he is not [considered] a new creation in the morning, he cannot recite the blessing of "Asher Yatzar" unless he relieved himself. There are opinions that if one urinated in the middle of the night, he may rely on the "Asher Yatzar" [the he will recite] in the morning. But this needs further analysis as far as what the halacha is, for there are many times that it happens that in the morning a person needs to urinate again, and the "Asher Yatzar" that he recites on the morning [urination] cannot count anymore for the nighttime [urination] - as is seen in Mishna Berurah 7:1. One who G-d fearing will be stringent on himself and wash his hands and put on pants, or wear something else that will make a separation between his heart and his genitals, and recite the "Asher Yatzar" blessing. But he should make sure to distance himself from the container of urine [at least] 4 cubits if it's in the same room as him. But if the container was not designated for this (ie. to be used for urine), it's enough to just add a revi'it of water to the urine every time one urinates into it.