'Mad Dog' Mattis sleeps well, keeps others awake

US Defence Secretary Jim Mattis.

Watch out for the "Mad Dog" when the sun goes down. That's the message on Sunday from Defence Secretary Jim Mattis, whose nickname has been used by President Donald Trump on Twitter.

The Pentagon chief is a battle-hardened, tough-talking retired Marine Corps general who was entrusted with some of the most challenging commands in the military.

So what might you expect him to say when asked, "What keeps you awake at night?"

Maybe another missile launch from North Korea? An attack by Islamic State militants?

An incident in the South China Sea? "Nothing," he tells CBS' "Face the Nation".

"I keep other people awake at night."