Why do hotel pillows turn black after sleeping all night?

The grease secreted from the face will rub against the pillow.

It may be oily skin or seborrheic dermatitis. People with oily skin will secrete a lot of oil on their scalp and face. If they don't clean for a long time or often, the accumulation of oil will easily lead to seborrheic dermatitis, and the pillow will naturally be polluted and oily. People with oily skin can often wash their faces and hair, and clean their heads with shampoo.