The human body tends to prioritize REM sleep over SWS or LNREM sleep. REM pressure builds fast, SWS pressure builds slow. There is no evidence for LNREM pressure existing, or if it does exist it builds the slowest of all. This is why we say your body does not require LNREM, which explains how it will edit it out of sleep and rearrange the order in which sleep stages appear to make sure you are getting the REM and SWS you need. We call this rearranging from sleep pressure ‘Sleep Stage Repartitioning’.
When people first begin to sleep less than it takes either SWS or REM to actualize (26 minutes or less) you will only get light sleep, as LNREM sleep is all you get in the first 26 minutes of your normal 90-minute cycle.
When we are in a severe enough sleep-deprived state the body will repartition sleep stages so that the nap begins with REM within the first few minutes.
If someone were to oversleep to a full-cycle at this point, this is what it might look like now:
People’s symptoms of sleep deprivation begin to disappear as REM restores their body and mind, and a nap stabilizes, but SWS deprivation begins to accumulate.
After about a week, SWS deprivation becomes a priority, and the body will force sleep longer than 20 minutes so that SWS deprivation is dealt with.
If you are doing a schedule that consists of no core sleeps, then begins the cycle of napping, then crashing, literally blacking out, not remembering anything as you are forced into SWS for long periods of time. This is why having a human helper when adapting is regarded as perhaps the only way to sustain a long term nap only schedule. With a human helper, we can be forced into action and the brain will slowly come out of the low-frequency state. Another way to break SWS is to have a fright or panic, something to make your body kick into fight or flight with a rush of adrenaline.
Assuming, now, that you are following a schedule with core sleeps, if you have a core placed at the right time relative to your circadian rhythm, then your SWS will not build at all. If you have a core placed at the wrong time, SWS pressure will build, and the SWS will begin to be repartitioned into your cores. If you have two cores, then most likely you will have more SWS repartitioned into one than the other.
Stages in sleep depend on Circadian Rhythm
Not all naps are identical especially once you are adapted to a schedule. As a rule of thumb, assuming a natural circadian rhythm, morning is the most likely time for REM, the evening is the most likely time for mixed stage sleep, the night is the most likely time for SWS.
Considering for a nap only schedule, this is especially true. Because the allocation of sleep time is so small, one would assume the body would dedicate a nap time to one stage or another. This is true, in a way, but think about how as adaptation stabilizes, it is because of a gradual drop in sleep pressure. If no one stage has more sleep pressure than the other then it will follow a circadian pattern.
Practice makes perfect
The more you practice a nap the better quality it will become, even if you are not sleep-deprived. A nap will repartition over time if you are not sleep deprived it can take a few weeks to several months for light sleep to turn to REM or SWS. You need to actually be able to fall asleep for it for the practice to be real, lying there for it (because you aren’t tired enough and you are at the peak of a BRAC) is not going to do anything.