It was just a throwaway line, a "tip for falling asleep" in an article by Jomay Steen in the Rapid City Journal about adapting your body clock to the changes caused by going off daylight savings time.
Eat or drink milk products, which have moderate amounts of carbohydrate in the form of lactose, which raises our serotonin levels and causes us to feel sleepy.
Really? It's the lactose in milk that causes this?
I found a similar statement on GoAskAlice, the health service question forum from Columbia University.
If you're having trouble falling asleep, try a small snack of carbohydrate-rich food. Warm milk may work for the psychological comfort, but also because milk contains a moderate amount of carbohydrate in the form of lactose (milk sugar).
Why does this work? Alice explains:
Carbohydrate-rich meals often increase serotonin levels. However, manipulating serotonin levels through food may be very difficult to achieve because serotonin's properties may have varying effects in different people. Some people may experience a temporary lift in mood after a carbohydrate-rich meal, while others may become relaxed or sleepy. Certain foods that increase serotonin levels aren't the healthiest choices either. Believe it or not, candy and sweets, which are simple carbohydrates, have the greatest impact, but the effect will only last 1 - 2 hours. Complex carbohydrates (rice, potato, pasta) may increase serotonin levels, but not to the same extent because the protein content of these foods might actually inhibit serotonin production.
Here's a brief explanation of the mechanism behind the effect of food on serotonin levels: after consumption of a carbohydrate-rich meal, the hormone insulin is secreted, which causes a lowering of the blood levels of most amino acids (the building blocks of protein), with the exception of tryptophan, which is a precursor to serotonin. When there are high blood levels of tryptophan in relation to other amino acids, it enters the brain at a higher rate, thus synthesizing more serotonin. To make matters more interesting, tryptophan is present in many protein-rich foods, which have been found to prevent serotonin production. So, you can see how intricate and complex this system is.
From that explanation, it's not at all clear why milk - which contains proteins equal to the lactose content - would work at all.
And even more than the qualifiers it already gives, the page reminds us that "The carbohydrate - tryptophan - serotonin pathway is simply a hypothesis at this point."
Soy milk also contains carbohydrates, though, and I've never heard of anyone recommending warm soymilk before bed. Does lactose make us sleepy? I'd say unproven at best for the time being.