I would like to drink coffee without sugar when I am tired night. Also, I always add milk in coffee!!
When you drink black coffee on an empty stomach, the oxalic acid in it is absorbed from your digestive tract and enters your bloodstream.
Eventually, it is excreted by the kidneys and exits into the urinary tract, where the phosphoric acid combines with calcium in the urine. This becomes a calcium oxalate stone.
One way to prevent this is to add milk to your coffee.