Four lengths

Why these four

Twenty and thirty both finish before a cycle would have; sixty lands in the middle of one; ninety is one whole cycle and ends where the wake-time pages would have ended it. That is a statement about the arithmetic and not about what any of them does to a person.

The four side by side

At the default 15 minutes to drop off, lying down at 1:00 pm.

The last column is why the drop-off matters more on these pages than anywhere else on the site. Ten minutes out on falling asleep is half of a twenty-minute nap and a ninth of a ninety-minute one. Against a night of five cycles, 7h 30m, it is about 2 per cent.

Against a working day's break

Lying down, the twenty- and thirty-minute naps fit inside the hour-long meal interval that South Africa's Basic Conditions of Employment Act gives most employees after more than five hours of continuous work, and the hour and the ninety minutes overrun it. A written agreement can shorten the interval to as little as thirty minutes, and at thirty neither of the short two fits. None of the four fits inside the United Kingdom's twenty-minute rest break, and United States federal law requires no lunch or coffee break. Each length's own page has its figures.