Four hours of sleep loss = a six-pack of beer?

This one is all over the net. People claiming that missing four hours of sleep is the equivalent of drinking a six-pack of beer.

If I drink a six-pack of beer, I'm unconscious.

So I did some looking, and I haven't found an original source to this quote. The earliest sources claim that missing four hours of sleep for four or five days in a row is equivalent to staying awake 24 hours, and that is equivalent to being legally drunk. I can see how that got conflated by the usual game of telephone into missing sleep one night having that effect, and thence to someone's idea of what "legally drunk" entails.

There is also a US Army document that claims "On 4 hours sleep, 1 beer can have the impact of a six-pack" (the image below). Again, a much less strident claim and a more credible one.

Surprisingly, a perusal of Snopes fails to reward me with an article on the subject. I would have thought they would be all over this one.

Gentle Readers, can you find me a source for this claim?