Sleep Sync and Vertical Hold

2008.01.22   prev     next

According to research conducted about 40 years ago at the Max Planck Institute in Munich, the human body is set for a 25-hour sleep cycle. That is, when humans are placed in a controlled environment, devoid of any cues about whether it’s day or night, they settle into a 25-hour sleep cycle.

Why 25? The day is 24 hours, and has been for millions, maybe billions of years. Whether dictated by Darwin or Design, wouldn’t humans be adjusted for a 24-hour day?

Damn Those Bunny Ears

Here’s a similar question: Those of us who are as old as me remember, when we were kids, trying to tune in a weak station on our TV and seeing the picture “roll,” as in the illustration below. Why did it do that?

The answer is simple: The raster-beam circuitry in an old, analog TV was designed to jump back to the top of the screen about every 1/30 of a second, just like the incoming video signal does. But — there’s no way to make the TV and the video signal precise enough to jump back to the top at exactly the same rate. What if the TV is resetting vertically at 99.98% of the rate that the video signal is? Or 100.02%? Then the two will slowly creep out of sync and you’ll wind up with the split picture depicted above.

To solve this, the engineers designed the TV to have a natural vertical sync time a little longer than the correct time — say, 1/29 of a second. But, the TV circuitry is also looking for something called the vertical sync signal, embedded in the incoming video signal, which tells the TV when its time to go back to the top. If the TV has nearly reached its 1/29-of-a-second vertical sync time, and it sees the vertical sync signal in the incoming video signal, it jumps immediately to the top and starts the raster process over. That way, the TV says in perfect sync with the video signal.

If the video signal is weak, and the TV can’t see the vertical sync signal, then it jumps back to the top after 1/29 of a second, and the picture, while viewable, rolls vertically due to the difference between the video signal (1/30 of a second) and the TVs built-in vertical frequency (1/29 of a second).

Note that this scheme would not work if the TV had a naturally shorter sync time than the video signal. If the TV was set to reset vertically every 1/31 of a second, then it would do that before the vertical sync signal arrived, and the picture would roll in the other direction (or be otherwise messed-up), even when the incoming video signal was strong. So, the TV’s natural vertical frequency must be a little longer (not shorter) than the incoming video signal.

Nighty Night

So it is the same, I think, with human sleep. The human body is hard-coded for a 25-hour sleep cycle (a little longer than a real day), but is also ready to wake up a little earlier than that, when cued by a sunrise or a spazzing rooster or some such thing. When humans are isolated from the sunrise (and all other time-of-day cues), their sleep schedule goes into a 24-day “roll,” not unlike a TV that can’t see the vertical sync signal.