What indicates the onset of Stage-2 sleep?

The correct indication of the onset of Stage-2 sleep is sleep spindles. Sleep spindles are sudden bursts of oscillatory brain activity that occur in the frequency range of 12-16 Hz (depending on the species). They are a hallmark of Stage-2 sleep and are believed to play a role in memory consolidation and other cognitive processes.

In the transition from Stage-1 to Stage-2 sleep, the presence of sleep spindles distinguishes the deeper sleep state that characterizes Stage-2. Additionally, K-complexes—large waves that can occur during Stage-2 sleep—also contribute to this stage's distinct pattern, although sleep spindles are more specifically recognized as key markers for its onset. Therefore, while K-complexes coexist with sleep spindles in Stage-2, sleep spindles are fundamentally recognized as the clear indicator of the beginning of this stage.