Add some milk for a good sleep

[Milo Masson]

Like the Taste I believe the reason people put milk in tea is simply because they like the taste. Tea contains caffeine in the same quantities as coffee (more or less!).
When milk or other fatty substances (ghee, yak butter etc.) are put in the tea the barbiturates bind to the fat and are then digested and absorbed by the body.
The effect is to perk us up with the caffeines while at the same time relaxing us with the barbiturates.
So milky tea provides a much better way of waking yourself up than a cup of coffee and will not impair your ability to go to sleep.