From the 'bladder and bowel community' web-site [Southam, Warwickshire, England]
"Hormones – your body makes a hormone at night which slows down kidney function while you sleep, so you normally make much less urine during the night. As you get older less of this hormone may be produced and so your bladder continues to fill at its daytime rate. This means you need to pass water more frequently.
"Circulation – the kidneys filter your blood as it passes through them and collect waste products and excess fluids from around your body. As the waste products and fluids are collected the kidneys produce urine, which then passes to your bladder, where it is collected.
"As you get older, your heart and circulatory system may become less efficient, so that some fluids are left behind in the body's tissues, especially around the ankles, because it is harder for the circulation to pump fluids from there. When you lie down at night it becomes easier for the circulatory system to work. The fluid is absorbed back into your blood and filtered out through the kidneys, which then send it to the bladder as urine. This means that your bladder fills as you sleep much more quickly than it normally would".
ADH
This hormone is a chemical messenger from the brain to the kidneys which tells them to slow down making urine. It's called ADH (anti-diuretic hormone).
One of the reasons most of us get a good night's sleep is that our ADH levels go up at night, so our kidneys produce less urine.
The Dry Night - BladderBowel.gov.au
www.bladderbowel.gov.au/assets/doc/drynight.html