Why does my mom act weird, only at night?

Only at night, not all the time but most of it, she acts strange. It isn't anything that is alarming or impeding her usual activities, but it is strange. She usually is easy to talk to and alert, but at night, late, she becomes meek and sheepish. Also she is not very good at keeping up a conversation. Physically, her balance seems to diminish, her knees seem to lock up and she stiffly lurches around. She overall seems to becomes almost senile and ultimately confused by the entire environment of our house. No one else, namely my dad or grandma, pay enough attention to know but it is very obvious. It worries me, but also frustrates me since she seems to become less mature then me. I don't know if this is psychological or what but I need answers. I have confronted her but that did little to nothing.