Why does my 81 year old aunt with dementia get confused in the late afternoon?

A fellow caregiver asked...

My 81 year old aunt has lived an extremely stressful life but has always been a very loving person. She has been diagnosed with dementia but has times of lucidity. Late afternoon is horrible for her as she does not recognize her own children and talks about her life years ago. I don't know why this occurs at the same time every day. Any ideas?