Naps
Naps are short sleeps (as opposed to cores), usually 20 minutes long, or 40 for pronaps. The idea behind them is twofold: To make the removal of homeostatic pressure effective, gaps between the naps need to be limited, they should ideally be somewhat evenly spaced, though slightly variable frequency with a larger gap before a core can help increase the pressure for it.Naps should also be placed with peak-aware times, allowing for more vitals of the desired type. REM naps are usually easier to wake from for most people compared to SWS naps.
For many, napping first needs to be learned as the body assumed all sleep to be cores by default.
Sometimes, napping during the day leads to napawares, which may not feel like getting any sleep - despite getting sleep. Learning to nap also means you may experience premature-wakes.
Length
30m naps are notoriously difficult to wake up from because you enter SWS around 25m in on average, and SWS can cause bad sleep inertia, zombie-mode, or worse, simply sleeping through your alarm in some extreme cases. It's also unhealthy to interrupt SWS. For these reasons, we usually recommend 20m over 30m for naps.Don't allow yourself more than 2m to fall asleep (ideally, stick with 20m total nap time). Once sleep deprivation starts to kick in, you will fall asleep within minutes, and if your naps are any longer you risk entering SWS. If you want to sleep longer, try aiming for a full cycle of 90m.
If you don't have the ability to take 20m naps, two 15m naps could replace one 20m. It is best to keep all naps the same length.
The absolute maximum length sleep that can be considered a nap (on almost all schedules) is 50 minutes. Beyond that, a core can be assumed because both vital stages will be recovered.
Theory
The ubersleep formula, originally described by Puredoxyk in her book "Ubersleep: Nap-Based Sleep Schedules and the Polyphasic Lifestyle", asserts that a nap can replace an entire sleep cycle of 1.5h. This is what created the MonoCore (then Everyman) and Uberman schedules.In truth, the core still has to undergo compression, as vital requirements do not total the 2h that the formula would suggest - they total 3-4h. Each individual vital, e.g. REM, which naps are meant to replace, needs about 90mins to 2h, but this leaves the other vital (e.g. SWS) off the table. And getting the sleep compression required to reach those theoretical 3-4h of TST is largely impossible, so deepening is required instead, which also has its limits and is very unreliable. Until about 4h of total core, the formula usually works though, with the core(s) being heavily compressed by that point.