睡眠深度与活动的频率有直接关系，频率越高，睡眠深度越浅。一般是以身体活动减少和感觉灵敏度降低作为衡量的指标。
HRV频谱中低频和高频成分的比值（LF/HF）随着睡眠深度的变化而波动。
The ratio of the low frequency to high frequency components in HRV power spectrum(LF/HF) fluctuates with the change in the sleep stages.
研究人员说，这发生在第一次深度睡眠期间。
This was during the first deep sleep period, the researchers said.
当研究人员在第二和第三晚重复实验室里的实验时，他们发现在深度睡眠时左脑不能以同样的方式受到刺激。
When the researchers repeated the laboratory experiment on the second and third nights they found the left hemisphere could not be stimulated in the same way during deep sleep.
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