Our 18 year old calico does the same thing.. but during the day and throughout the night. We were told it may be feline senility. She is not in pain. She does not hear nor see well anymore either. We will try to deal with it as long as we can.
Good Luck. You are going to need it.
There is lots on google on this: search: feline senility
http://www.fabcats.org/owners/elderly/senility.html
The most commonly seen changes include spatial (space) or temporal (time) disorientation, altered interaction with the family, changes in sleep-wake cycles, house-soiling with inappropriate urination/defecation, changes in activity, and/or inappropriate vocalisation (often displayed as loud crying at night