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Why do adults spend much of the night in N2 sleep?

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Adults spend a significant portion of the night in N2 sleep—accounting for approximately half of total sleep time—because it is a primary component of the NREM sleep cycles that repeat throughout the night.12

While the specific functional purpose of the N2 stage remains unclear, its high prevalence in human sleep is notable, especially as it is only present in rudimentary forms in subprimate species. The high proportion of N2 sleep in adults is largely attributed to the progressive reduction and eventual loss of stage 3 (slow-wave sleep) in the NREM portion of sleep cycles as the night progresses.12

During N2, the brain produces sleep spindles and K-complexes, which are thought to help stabilize sleep by blocking sensory input and may assist in memory consolidation by facilitating the transfer of information to long-term storage. These transient activations during N2 may also serve to reinforce neural connections, helping to optimize daytime cognitive functions such as learning and attention.134