Sleep pressure

Sleep pressure is the technical term for what is essentially tiredness - but not quite. As the name implies, sleep pressure is the pressure our brain feels to sleep, the higher, the more you want to sleep. However, there isn't just one sleep pressure, there are three.

Anystage pressure

Generalized sleep pressure, also known as just homeostatic pressure, is the weakest kind of pressure, and does not cause a rebound; but this is what you usually associate with tiredness. It is what you feel after skipping a sleep, or when sleeping too little short-term in general. This kind of sleep pressure is regenerated by any sleep stage, which is why naps can always reduce it, even if they don't contain any vitals or are even just napawares.

This kind of sleep pressure is hypothesized to be related to Adenosine buildup in the brain.

However, as sleep debt accumulates during adaptation, REM pressure takes over:

REM pressure

REM pressure is the worst-feeling one mentally - when it is high, you struggle to maintain memories or stay focused, you may even hallucinate, and it is often very hard to stay awake, as REM p. most readily causes microsleeps. REM pressure is however required to initiate SOREM. This kind of pressure takes longer to accumulate than LS-Pressure, and it's what you start feeling in stage-2 and stage-3.

SWS pressure

SWS pressure is a silent killer for adaptations, as the high amount of SWS presence in sleep caused by it can easily cause an oversleep because it is extremely potent for triggering zombie-mode, crashes, and even blackouts (unnoticeable microsleeps). But as the silent implies, it is not as easy to notice as REM pressure, and it takes longer to accumulate. It does not cause the typical feelings of tiredness, but rather a persistent fatigue, usually physical, along with sluggishness, aches, eyebags, and extreme weakness. In general, any "physical" tiredness is usually a lack of SWS, and it is a good idea to prepare extra hard for a zombie-mode wake if you notice it: Set alarms, many of them, and make sure to go to sleep FULLY intending to wake up and get up immediately tomorrow. SWS pressure does not make you feel conventionally tired though, it just feels like your body is struggling.

Cycles

Despite the alarming symptoms, SWS pressure is extremely potent at equalizing itself. If you don't sleep, it will make you, and if you already repartitioned REM/SOREM everywhere, it will readily rip that back out and replace it with SWS. This causes a cycle during stages 2 and 3 (and possibly 4) where you oscillate between feeling tired (REM pressure, when SWS is dominant) and feeling lethargic but mentally perfect (SWS pressure, when REM is dominant).

As stage 3 and 4 progress, these cycles will decrease in strength, and disappear completely once repartitioning is complete.