To project, a sleeper is placed into a massive tank called a cradle. The temperature in these tanks are carefully lowered as the sleeper's blood is pumped out of their body and replaced with a special fluid made to keep the organs healthy and the body from decaying. The chemicals, however, interrupt the body's natural activities and cause the projector's heart to stop beating, a process called flatlining. Eventually, the temperature in the tank reaches -80 degrees Fahrenheit, further slowing decay. It takes around five hours for a person to be flatlined, and almost as many to be thawed out and revived.
Generally, sleepers are not sleepers by choice. They are projectors that, for some reason, cannot project via skimming. There are several reasons; a lack of near-death experiences, bad reactions to the drugs used by skimmers, or a deep fear of skimming. There are rare times, however, where a projector opts to become a sleeper. Most common of these are projectors with a terminal illness or disease. The constant near-death state that the body of a sleeper goes through seems to retard the progress of cancer or other deadly diseases and disorders. Some sleepers also feel uncomfortable in their body because of a physical handicap, and enjoy the mobility that being a sleeper offers, since sleepers can remain out-of-body for longer periods of time than skimmers.
Unlike skimmers, sleepers do not have a silver cord connecting their souls to their bodies. Because they lack the cord, a sleeper is actually closer to being a true ghost than their meditating counterparts. This also means that damage is not sent back to their bodies if they are injured. In fact, the only ways a sleeper's body can be harmed is if they send Spite back to their body to be healed like regular damage, or if it is attacked directly in the cradle. Unfortunately, the lack of contact with their bodies means they also cannot recover Vitality while in ghost form.
It should be noted that sleepers are the earliest form of living projector; it is because of the clients and work done by JDG Cryogenics that the ability to project while frozen was discovered. All of the initial projection experiments were done with sleepers, including the infamous Project Flatline.