Which of the following is true of a night's sleep?

Question

BlackTom AI · Accepted Answer

Slow-wave (deep, N3) sleep is concentrated in the early part of the night because homeostatic sleep pressure and slow-wave activity are highest after wakefulness. Adult sleep cycles (~90 minutes each) shift across the night: NREM stages (especially N3) dominate early cycles, while REM periods lengthen in later cycles.

Relevant knowledge points:
- Sleep architecture cycles roughly every 90 minutes through stages N1, N2, N3 (deep/SWS) and REM.
- N3 (deep slow-wave sleep) is most abundant during the first few cycles; REM duration increases across successive cycles.
- Typical adult distributions: N1 ≈ 2–5%, N2 ≈ 45–55%, N3 ≈ 15–25%, REM ≈ 20–25% of total sleep (values vary).

Step-by-step reasoning:
1. A typical 8-hour (480 min) sleep with N3 ≈ 20% yields ~96 minutes of deep sleep overall. Those ~96 minutes are not evenly split; instead they are concentrated early. 
2. Early night: first two to three 90-min cycles contain the bulk of N3. For example, the first half of the night (first 240 min) might include 60–80 of the 96 deep-sleep minutes, while the second half (last 240 min) contains far less (often only 10–30 minutes), because later cycles emphasize REM and lighter NREM.
3. Physiological basis: synaptic downscaling and high sleep pressure produce large-amplitude slow waves early; as pressure dissipates, N3 diminishes.

Why the correct answer is correct:
- “Deep sleep is much greater in the first half than in the second half of a night's sleep” matches known sleep architecture and observed distributions across cycles.

Why other options are incorrect:
- “REM sleep decreases as the night progresses” is false — REM increases in duration later in the night.
- “Individuals normally spend about 90 percent of sleep in light sleep” is incorrect — light sleep (N1+N2) is substantial but not 90% (N2 alone is ~45–55%).
- “Most REM sleep takes place during stage N1” is incorrect — REM is a distinct state, not part of N1.

Conclusion: Deep sleep is indeed much greater in the first half of the night, confirming the provided correct answer.

Which of the following is true of a night's sleep?单项选择题

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