The article, which is referred to specifically in the Daily Mail piece, says something more precise. It refers to an association between the disruption of circadian clock in mammals and interference in the regulation of cell cycle and malignancy, and it says:
"The molecular intracellular signalling pathways by which light regulates and modifies the expression of the cell cycle in tumorigenesis genes is not yet clear. The increased relative risk for cancer among shift workers raises the question of whether this is a causative phenomenon or spurious association."
But it says nothing about increasing cancer by flicking on light switches when one is going to the toilet.