Back in the olden days in the 1960s...when they only had UHF and VHF....everynight they would go off the air at midnight or so......I was a wee pup...I remember those days well........:-)
They still did that into the '70s, then came cable. After a couple years when it caught on, most 'sign offs', signed off. ;-)
I'm older than you by far ... I remember too. The test pattern Indian. But TV has been 24 hours for so long. :)
Yep, thet had to shut down for almost nightly maintenance and let those giant 5 foot tall transmitter tubes cool down. Then they would sign back on about 5 AM with the ubiquitous Indian Chief test pattern and later the color adjustment bars so's you could adjust your color teevee to NTSC (Never The Same Color) standards, ad long as you could figger out what the hell "magenta" and "cyan" were.