熟睡

1. Origins, structure, and basic meaning

• Structure: Written 熟睡 and read じゅくすい, it is sleeping deeply and continuously enough for thorough rest.
• Basic usage: It appears in an unusually sound eight-hour sleep and sleeping until morning after changing pillows.

2. Common pitfalls and similar expressions

• Depth and quality, not duration alone
  • A long time in bed does not necessarily mean the sleep itself was deep.