Unfortunately, short of sitting in a personal anechoic chamber, nothing will completely isolate you from sound, due to the physics of human hearing. You hear most of what you hear by means of your ears. The Sleeep® and Sleeep® PRO work by blocking sound from entering your ear canals. However, sound will still vibrate your skull, cheek bones and jaw, albeit vastly less than sound entering your ears would. This is called bone conduction.
What Sleeep® earplugs do (when correctly fitted) is they 'turn the volume of the world down drastically', So its not 100% silence, but certainly enough to ensure that neighbors who go bump in the night don't wake you up!