Cows milked at night produce milk that makes people sleepier.
A study at the Uimyung Research Institute for Neuroscience in South Korea claims the milk taken from the farm animal during late hours contains "exceptionally rich amounts of tryptophan and melatonin," the Daily Telegraph newspaper reports.
The research team said: "Milk has long been known and used to promote sleep. The sleep-promoting effect of milk has been attributed to its psychological associations (i.e. the memory of a mother giving milk at bedtime) and its rich store of sleep-promoting components."