A 20-minute nap
Twenty minutes is under a quarter of a ninety-minute cycle, so it is over long before a first cycle would be.
| Stage | Time |
|---|---|
| 2:00 pm | you lie down |
| 2:15 pm | asleep, give or take |
| 2:35 pm | the alarm goes |
That is 35m of lying down for 20m of sleep, allowing 15 minutes to drop off.
The four lengths from the same moment
| Nap | Up at | Lying down for |
|---|---|---|
| 20 minutes | 2:35 pm | 35m |
| 30 minutes | 2:45 pm | 45m |
| 60 minutes | 3:15 pm | 1h 15m |
| 90 minutes | 3:45 pm | 1h 45m |
What the length is
Twenty minutes does not divide into a cycle and is not meant to. The nap ends long before a first cycle would, so there is no cycle boundary for the alarm to land near, and the sum on this page is a straight one rather than a count of cycles.
The arithmetic is the shortest on the site. Add the minutes it takes you to drop off, add twenty, and that is the alarm. The drop-off allowance is the larger source of error here — on a twenty-minute nap, being ten minutes out about falling asleep is a fifty per cent error on the sleep itself, which is not true of any other length on this site.
A nap is counted straight, not in cycles. The only assumption borrowed from the rest of the site is how long it takes you to fall asleep, and it is the same number you set on a wake page. Nothing here says a nap of any length will do anything for you.
Against the break rules
At the default fifteen minutes to drop off, a twenty-minute nap takes thirty-five minutes lying down. A worker in the United Kingdom is entitled to a twenty-minute rest break on a working day of more than six hours. The nap is fifteen minutes longer than that. South Africa's Basic Conditions of Employment Act gives most employees an hour's meal interval after more than five hours' continuous work, and the thirty-five minutes fit inside it with twenty-five to spare. A written agreement can cut the interval to as little as thirty minutes, and then the nap no longer fits. United States federal law requires no lunch or coffee break.
The other three
A whole night instead
Past about three hours a nap is a short night, and short nights are counted in cycles on the wake-time pages, one for every quarter hour of the day.