Daniel Pink has an excerpt from his new book When: The Scientific Secrets of Perfect Timing, that talks about the rather large effect circadian rhythms have on our daily performance:

Afternoons are the Bermuda Triangles of our days. Across many domains, the trough represents a danger zone for productivity, ethics, and health. Anesthesia is one example. Researchers at Duke Medical Center reviewed about 90,000 surgeries at the hospital and identified what they called “anesthetic adverse events”— either mistakes anesthesiologists made, harm they caused to patients, or both. Adverse events were significantly “more frequent for cases starting during the 3 p.m. and 4 p.m. hours.” The probability of a problem at 9 a.m. was about 1 percent. At 4 p.m., 4.2 percent. In other words, the chance of something going awry was four times greater during the trough than during the peak.

I think everyone has been in those after-lunch meetings where the room gets heavy, and people struggle to stay alert and pay attention. This is not just a function having had one too many martinis with lunch: it’s a universal thing. Yet the modern workday is organized with the assumption that every hour is identical to every other, and that we operate with the same level of effectiveness on any kind of task whether it’s 9:01 AM or 4:58 PM.

Indeed, in my study of companies that have shortened office hours, one of the consistent things they do is craft the workday to better match people’s daily rhythms: they let people work on their most important tasks when they’re most energetic, and put off less significant things (and many meetings) until later in the day, when you might have less energy and attention– but you might also need to spend less, as well.