Figure 7-1. Sleep stages. The N4 stage, according to modern classification, is combined with N3.
[Source: Krejcar et al, 2011]
A night's sleep consists of several 90-120 minute cycles, each is divided into four stages: N1, N2, N3 and REM (Figure 7-1).
Here "REM" stands for "rapid eye movement" and "N" for "non-REM" (NREM).
- N1: At this stage, the alpha (α) activity begins to decrease in most cortical areas, except the frontal lobe.
- N2: This stage is characterized by sleep spindles and K complexes in the EEG (Figure 7-2).
- N3: This is the deep sleep stage, characterized by slow oscillations, thus also called "slow wave sleep".
- REM: At this stage, the sleeper may perceive internal brain activities, namely, "dreams."
Figure 7-2. Sleep spindle and K complex. [Source: Wikipedia] p>