The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
Appendix I
Indo-European Roots
ENTRY:
swep-
DEFINITION:
To sleep. 1. Suffixed form *swep-os-.sopor; soporific, from Latin sopor, a deep sleep. 2. Suffixed form *swep-no-. somni-, somnolent; insomnia, from Latin somnus, sleep. 3. Suffixed zero-grade form *sup-no-. hypno-, hypnosis, hypnotic, from Greek hupnos, sleep. (Pokorny 1. sep- 1048.)
ep- 1048.) 
