There was a time in the recent past when TV channels, including cable TV channels like HBO, where not a 24 hour event. People just didn’t watch TV late enough to justify running programming. So the channels would sign off for the night and then sign on in the morning. What they did in those in between hours varied by channel. Some just showed a test pattern or snow, while others would use it in a more productive way, say as a guide for what would be on the next day. Not a lot of channels sign off nowadays, but I imagine a few still might. These videos are from the HBO sign-on and sign-off in the 1980s, when HBO signed off around 2 am.
I would like to say that as a young child, I was not up at this time. Sadly though, once we got cable in the house, I was a non-sleeping monster. When I got to see the HBO sign-off, I felt like I had defeated the day. Those few times I stayed up until HBO Sign-On, I felt like I had defeated the night. That I was seeing something that few people got to see.
Once we got a VCR, the thrill was gone, but I can always look back on these videos with the nostalgia that came with poking around edges of adulthood in the clueless way that kids do.