A friend fell asleep and fell out of his stand. Woke up in blinding pain. The front of his shirt looked like a pup tent because, aside from cracked ribs, dislocated shoulder, etc., he split his sternum down the middle! He's lucky it went out instead of in. By the way, you don't "set" a sternum back in place. You wait for it to go back on it's own, over many painful weeks. He didn't remember how he was able to somehow crawl back to his car and drive to a farm house for help.
A harness is way cheaper than the hospital visit, missed work, or a funeral. Keep that in mind.