Why is our dog torturing us?

We have a 7 year old Corgi that, as of last Friday, decided that he would like to bark all night long while we're sleeping for no reason at all.

The first night it happened, we got up in a rush to let him out to the bathroom, thinking that was why he was suddenly barking. He didn't need to go, he just kind of wandered around the yard and then came back in very nonchalantly. So, we put him back to bed. The second I got back into bed he started barking again. We got up two more times to let him out that night, because he has never done this before and we were worried about what could possibly be wrong. Each time he just got out of his kennel and wandered around the house a bit before laying down in the livingroom and staring at us. As soon as we were up for the day, he was fine (and also completely ignoring us in another room, so it's not like he wanted to hang out or needed attention for some reason).

He's done it every night since. We wore earplugs last night and just completely ignored him, not coming out of our bedrooms once to check on him or let him out. We thought he would eventually realize no one was coming and shut up, but he didn't. He literally barked the ENTIRE NIGHT. And even with the earplugs, neither of us got any sleep before having to go to work this morning.

What the fuck has gotten into him? Has anyone experienced this? Any tips or ideas? This is completely out of the blue, I cannot make any sense of it.