When you first drift off into slumber, your eyes will roll about a bit, your temperature will drop s

When you first drift off into slumber, your eyes will roll about a bit, your temperature will drop slightly, your muscles will relax, and your breathing will become quite regular．Your brain waves slow down a bit, with the alpha rhythm predominating for the first few minutes．This is the first stage of sleep．For the next 30 minutes or so, you will drift down through Stage 2 and Stage 3．The lower your stage of sleep, the slower your brain waves will be．About 40-60 minutes after you lose consciousness, you will reach the last stage．Your brain waves will show the delta rhythm．You may think that you stay at this deep stage all the rest of the night, but that turns out not to be the case．About 80 minutes after you fall into slumber, your activity cycle will increase slightly．The delta rhythm will disappear, to be replaced by the activity pattern of brain waves．Your eyes will begin to dart around under your closed eyelids．This period of Rapid Eye Movements lasts for 8-15 minutes and is called REM sleep. During both light and deep sleep, the muscles in your body are relaxed but capable of movement．As you slip into REM sleep, a very odd thing occurs—most of the voluntary muscles in your body become paralyzed．Although your brain shows very rapid bursts of neural activity during REM sleep, your body is incapable of moving．REM sleep is accompanied by extensive muscular inhibition. On the part of an average sleeper, there ______ of sleep in cycles. When you first drift off into slumber, your eyes will roll about a bit, your temperature will drop slightly, your muscles will relax, and your breathing will become quite regular. Your brain waves slow down a bit, with the alpha rhythm predominating for the first few minutes. This is the first stage of sleep. For the next 30 minutes or so, you will drift down through Stage 2 and Stage 3. The lower your stage of sleep, the slower your brain waves will be. About 40-60 minutes after you lose consciousness, you will reach the last stage. Your brain waves will show the delta rhythm. You may think that you stay at this deep stage all the rest of the night, but that turns out not to be the case. About 80 minutes after you fall into slumber, your activity cycle will increase slightly. The delta rhythm will disappear, to be replaced by the activity pattern of brain waves. Your eyes will begin to dart around under your closed eyelids. This period of Rapid Eye Movements lasts for 8-15 minutes and is called REM sleep. During both light and deep sleep, the muscles in your body are relaxed but capable of movement. As you slip into REM sleep, a very odd thing occurs—most of the voluntary muscles in your body become paralyzed. Although your brain shows very rapid bursts of neural activity during REM sleep, your body is incapable of moving. REM sleep is accompanied by extensive muscular inhibition. On the part of an average sleeper, there ______ of sleep in cycles.重试 错误原因 A.重试 错误原因 is one stage重试 错误原因 B.重试 错误原因 are two stages重试 错误原因 C.重试 错误原因 are six stages重试 错误原因 D.重试 错误原因 are four stages重试 错误原因

我们根据文章内容来梳理一下睡眠阶段的描述：

入睡时是 Stage 1（第一阶段），α 波主导。

然后约 30 分钟里，会逐渐进入 Stage 2 和 Stage 3。

大约 40–60 分钟后，到达 last stage（最后阶段），呈现 δ 波（delta rhythm），这是深睡阶段（Stage 4，如果按传统的 4 阶段分法，这里的“最后阶段”应该是第 4 阶段或第 3+4 阶段，但文中称 Stage 1→2→3→last stage，所以可能是 4 阶段分法）。

约 80 分钟后，会出现 REM sleep（快速眼动睡眠），δ 波消失，脑波变得活跃。

文中明确提到 “during both light and deep sleep”（浅睡和深睡阶段）以及 REM sleep，这意味着睡眠包括非 REM 阶段（有深浅不同阶段，包括 Stage 1–4）和 REM 阶段。
但问题问的是 “On the part of an average sleeper, there ______ of sleep in cycles.” 是在问一个周期里有几个阶段吗？
其实传统的睡眠周期是：
非 REM 睡眠（4 个阶段：Stage 1, Stage 2, Stage 3, Stage 4，或现代分为 N1, N2, N3）加上 REM 睡眠。
如果按文章分：Stage 1 → Stage 2 → Stage 3 → last stage（= Stage 4），然后 REM sleep，所以一个周期中睡眠阶段的数量：文章中列出了 Stage 1, 2, 3, last stage（4），加上 REM sleep 是 5 个“不同状态”吗？
看选项：A: 1 stage，错；B: 2 stages，错；C: 6 stages，错；D: 4 stages。
但文章如果把非 REM 当作从 1 到 last stage 算 4 个 stages，再加 REM，是 5 个阶段，不在选项中。若把 3 和 4 合称为深睡阶段（Stage 3+4），就是 Stage 1, Stage 2, Stage 3+4, REM sleep，这样是 4 个阶段。这与选项 D 相符。
文章里说 “for the next 30 minutes or so, you will drift down through Stage 2 and Stage 3. The lower ... About 40–60 minutes after ... you will reach the last stage” ，说明 Stage 3 和 last stage（Stage 4）是分开提的，但有些教材将 Stage 3 和 4 统称深睡（慢波睡眠）。
不过，问题可能是在问一般所说的睡眠阶段数（经典是 4 个非 REM 阶段+REM=共 5 个阶段，但常见选择题会归纳为 4 个阶段：1, 2, 3, REM，或者 1, 2, 3, 4, REM 中的 4 是周期数？不，这里问 “there ______ of sleep in cycles” 可能是指一个周期包含几个阶段。通常一个睡眠周期（约 90 分钟）包含：
Stage 1 → Stage 2 → Stage 3 → Stage 4（或 N3）→ 短暂回到 Stage 2 → REM sleep。但是，如果从大的划分来看，一个睡眠周期主要包括：
N1, N2, N3, REM 四个部分（其中 N3 包含原来的 Stage 3 和 Stage 4）。

从文章看，叙述了 Stage 1, Stage 2, Stage 3, last stage (Stage 4)，然后 REM，这意味着至少提到 5 个不同状态，但选项最大是 6 stages（C）和 4 stages（D）。
我们再看原文最后一段与第一段的结合，能看出：
第一阶段（Stage 1） → Stage 2 → Stage 3 → last stage（= Stage 4） → REM sleep。
这是 5 个阶段。但选项里没有 5，因此必须判断命题者意图——若按照经典旧分类法，睡眠阶段（stages of sleep）分 5 期（1,2,3,4,REM），但 3 和 4 常合称 Delta sleep，所以有时也叫 4 个主要阶段（1,2, 慢波睡眠, REM）。
文中的 “last stage” 是指 Stage 4，所以是 4 个非 REM 阶段加 REM = 5 阶段，但选项 D 是 “are four stages” 指非 REM 的四个阶段吗？题干里 “there ______ of sleep in cycles” 可能是 “there are four stages of sleep in cycles”。

再检查上下文逻辑：
“About 80 minutes after you fall into slumber...” 之后进入 REM，说明一个周期约 80–90 分钟，周期内包含：
Stage 1（几分钟）→ Stage 2（10–25 分钟）→ Stage 3（几分钟）→ Stage 4（深睡，30 分钟）→ REM（10 分钟），
所以按阶段分：Stage 1, 2, 3, 4, REM 共 5 个阶段，但题干若说 “an average sleeper” 的一个睡眠周期内有几个阶段，若按现代标准答案（N1, N2, N3, REM 四个阶段）选 D。
结合此题是选择题且四个选项，D 是合理且常见的答案，因为 Stage 3 和 Stage 4 常被视作同一深睡阶段。
文章明确分开 Stage 3 和 last stage，但题目要求可能只是计算主要的“阶段”数时，将 Stage 3 和 Stage 4 合为“深睡阶段”，加上 Stage 1, Stage 2, REM sleep 共四个阶段。

答案：D. are four stages ✅

所以选择 D。