The figure of eight splint is designed to hold the shoulders back in a military brace (or at attention) position. This has the clavicles pulled out to their maximum lengths, with no pressure applied along their axis.
However, if it is not applied very tightly, pulling the shoulders back, then it is no more effective than a sling. Most of the time, figure of eight splints are usually used in children, but in adults, a sling is most often all that is necessary.
As to sleeping, it is usually recommended that the patient sleep in an upright position for the first 7 to 10 days. This can be in a lazy boy type chair, or propped up in bed on several pillows. The one position that is usually not good, is to lie on the side, as this causes the shoulders to round forward, placing pressure on the fracture site.