When one of my cat doesn't used her litter tray at night?

[Harvey]

This started out as a new behaviour for one of my female cats. She used to make use of her litter tray all the time but I'm not sure what happened in the past month for her to stop using her litter tray at night. It's not like the litter tray is dirty because I make sure to keep it clean at all times.

What could be the cause of this new behaviour?

 

[Foxybananas]

Maybe her bladder is weaker and she can't hold it? 🤔 Is she using it in the daytime? If so, maybe she's too tired to make it to the litter box at night.