A continuous 8 hour work day is a relic of the past. It makes sense for physical labor and manufacturing work, but with information workers it doesn’t account for the mental energy cycle. The ability of a factory worker to think analytically is irrelevant, he’s either cranking widgets or he isn’t.
In the case of the modern information worker, nearly all tasks involve creative or strategic thinking. The way someone answers an email or interprets a piece of information can differ drastically depending on his or her energy level. Nobody does their best work 5:30 in the afternoon after they’ve been sucking down coffee all day to stay awake.
Now, I do have the ability to work, fully concentrated, for 18-hour stretches (once I did a 40-hour stretch!), and I don't get anxious like John says he does in his article. However, that fact only reinforces the idea that 9-to-5 simply doesn't work for information workers.
Information technology leaders clued in on this a long time ago: witness the 24/7 continuous operation and open-door policy of leading information companies, like Google.
It's only a matter of time before people in the IT sector start acknowledging a 9-to-5 shift as a reason not to work at companies who practice it.