Kidney function, including fluid filtration and regulation via hormones like vasopressin, is significantly reduced approximately 10 hours after waking.

“Well, it turns out that the filtration of fluids from your body, which is accomplished of course, by your kidneys and by way of your bladder, and the excretion of fluid out urethra, so-called urination, is strongly, strongly circadian dependent, meaning the cells of your kidney and the cells even of your gut, in fact, all the cells of your body, but especially the cells of your kidney, which filter the fluid that comes into your body, and that makes certain hormones like vasopressin, which is antidiuretic hormone. All of that functioning of the kidney is under strong regulation by so-called circadian clock genes, circadian clock genes are genes that are expressed in every cell, but that in certain cells of the body very strongly impact whether or not that organ, in this case, the kidney is going to be activated, meaning functioning at a very high level or at a reduced level.”