Most of us are familiar with television's 9pm threshold -that witching hour which separates family viewing from the sex, violence and dirty language the rest of the population enjoys - but who's heard of the 5am threshold?
Channel 4, which in the new year launched a 24-hour all-night schedule, will have to watch this new cut-off point closely, since some of its all-night programming ideas may prove controversial.
Channel 4's decision is not unexpected. For the past decade its programmes have been slowly eating into the wee small hours, to the extent that by last December it was off the air only for a couple of hours a night.
What is new, however, is the idea of a regular, themed, late-night schedule. Tuesday nights, for example, will be devoted to indie cinema, with double bills of Jim Jarmusch and Atom Egoyan. A season of maverick American director Sam Fuller's films will be introduced by a documentary about him by Tim Robbins. And kicking off this weekly all-night moviethon will be a new half-hour cinema programme called, originally enough, Film Night.
Wednesday nights will mean red eyes for sports fans as three hours of live sport hits the screen, led by NBA basketball and Superbowl American football. For those who like sport with attitude, snowboarding will raise the odd goosebump or two while the rest of the nation snores. If you can keep your eyes open long enough, Football Dreams will show what happens when young hopefuls join Chelsea Football Club as YTS trainees.
By Thursday night, most night owls will be flagging, leaving only shift workers and security guards to catch a night of factual programming, details of which are still under wraps but which includes The Weekly Planet, a live discussion headed by Jon Snow.
Channel 4's venture into 24-hour broadcasting is due to its success in attracting upmarket audiences - and thus lucrative advertising revenue - and new tax breaks granted by the Government. Still, graveyard-shift viewing on commercial channels remains very low: between 3am and 4am the number of viewers drops to fewer than 500,000.
A week of bleary-eyed late-night television probably means that by Friday you've had enough and it's bed by 10pm. But don't forget to set your video, because Friday night is billed as cult-movie night. With politicians still getting in a lather about sex and violence, all-night weekend broadcasting may be one way in which programmes avoid the censor.
One day soon, deep in the depths of the night, you may even be able to watch uncut versions of all those films that routinely have chunks sliced out of them when they get screened during peak hours. Just as long as all the naughty bits are tucked out of sight by 5am.