Frequent nighttime waking to urinate can also be a symptom of sleep apnea.
From Steven Park's (apnea doc's) website:
We’ve know for many years that frequent bathroom trips at night are not due to bladder or prostate problems, but mostly due to breathing pauses that make you wake up and and think that you have to go. Numerous studies have shown that most men and women who have to go to the bathroom often at night or have overactive bladders during the day actually have sleep apnea.
A new study confirms these findings again, but points out that the presence of frequent trips to the bathroom is as sensitive in predicting obstructive sleep apnea as snoring!
Sleep apnea also makes the body produce more urine, and because you wake up every few hours, you’ll want to go, but you won’t have a completely full bladder. Sudden shifts in blood flow into the heart after an obstructive event increases production of atrial natriuretic peptide, which makes you produce more urine. One study found that diluted levels of urine during these episodes is also predictive of having obstructive sleep apnea.
Links to studies found here: http://doctorstevenpark.com/sleep-apnea-and-nighttime-urination