From the Google India blog:

We're always working on ways to make the YouTube offline experience even better. Many mobile operators already offer discounted data and promotions to their subscribers, for the Internet during late night hours. Today, we're beginning to roll out a feature called Smart Offline on YouTube that lets you takes advantage of these cheaper data rates as offered by your mobile operator, without requiring you to stay up late at night.

Here's how it works: when you tap on the familiar grey arrow to take a video offline, a prompt will appear, giving you the option to select "Save overnight" using your mobile operator's discounted night data plan. When you select it, the YouTube app will smartly schedule your video to be offlined after peak hours that night. When you wake up the next morning, your videos will be ready for you to watch offline and on the go, with no buffering.