There are two words rendered "Sleep":—
  1.  katheudo = to compose one's self for sleep. Occurs twenty-two times; never used of death.
  2.  koimaomai = to fall asleep (unintentionally). Hence this latter is used of death, as it is involuntary,

while katheudo is voluntary. See this difference illustrated in 1Thessalonians 4:14 (where it is koimaomai), and 5:6, 7, 10 (where it is katheudo). Occurs eighteen times; always of death, save Matthew 28:13. Luke 22:45. John 11:12. Acts 12:6.