Brain Injury, Patient Combative At Night?

it's called sundowning

My friend's mother recently survived a brain aneurysm. At first her recovery seemed miraculous. She appeared to have retained most all of her faculties and her personality. She went straight from intensive care to rehab. Here's the problem, though. During the day she acts and speaks normally, like her old self, but come night it's as if "a switch has been flipped". She becomes a different person, very combative, very agitated. My friend says the change is shocking and goes on almost all night...until the sun comes up and her mother is 'normal' again. Does anyone have any idea what this is, what it's called? My poor friend is at her wits end. Thanks for your help.