A special bag, that completely encloses a person, from head to toe, as a form of mummification. To create a form of sensory deprivation, that many consider erotic.
A 'sleep sack' is similar to an ordinary sleeping bag, however, it does cover one totally, in a very restrictive manner. Use of belts, straps, or rope, on the outside, further enhance the feeling of enclosure.
The goal is to completely cover the subject, in a tight restricting manner, that isn't just about being tied up, but about being totally encased in a wrap of some kind.
Saran Wrap is commonly used, but a sleeping sack, is another close favourite.
The use of a hood is important in making the sensation total.
Many associate the feeling of mummification, as being in a sensory deprivation chamber, though less intense. This should not be taken to mean one leaves the subject alone, while in a sleeping sack.
The thrill or idea, is to create a sort of out of body feeling, of floating, where all other sounds, other than your own body sounds, are inaudible.