CG and his vibrator

I am getting to know new flatmate. About one hour every evening. I leave in the morning, he sleeps. I come home in the evening, he's away. The routine we have established includes him coming home around 11pm and we have a talk before going to bed. This has now happened three nights in a row, so it's safe to say it's become a habit. Then I go to bed with my book, and he goes to get ready for bed. This includes a vibrating sound from the bathroom. At first this was a bit puzzling until I realized he has an electronic toothbrush. Phew, what a relief! But why does the vibrating sound go on for 5 minutes? How dirty can his teeth be? There are surely things still to be discovered about this new man in my household...