The problem is extremely common among older men because it can be caused by an enlarged prostate (benign prostatic hyperplasia). The prostate presses on the tube called the urethra and prevents the bladder from emptying completely - hence the need for more frequent trips to the toilet.

Other causes of nocturia:
Swollen legs associated with a heart condition.
Taking a diuretic medicine that causes more urination.
Having a different 'body clock', which causes a normal daytime urination pattern to occur at night.
If the bladder is unstable or irritated as a result of cystitis.
Diabetes, if it is not controlled. This leads to sugar appearing in the urine, which stimulates extra urine to be produced.