Lactose was a form of sugar and carbohydrate that was found in milk. It consisted of galactose and glucose.
Milk was often used as a home remedy for insomnia, as the amino acid in the heated lactose was believed to act as a natural sedative.
Captain Jean-Luc Picard's aunt Adele had a recipe for a steamed hot milk toddy, which Picard shared with Doctor Crusher. She in turn prescribed one to Commander Riker, explaining that the lactose in the beverage would help him sleep better. (TNG: "Cause and Effect", "Schisms")