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 1Thess. 4:14 (where it is koimaomai), and 5:6, 7, 10 (where it is katheudo).  Occurs eighteen times; always of death, save Matt. 28:13.  Luke 22:45.  John 11:12.  Acts 12:6.