You should sleep with a sling on in some cases such as after having shoulder surgery, after dislocating your shoulder or after you broke your arm or elbow.
Sleeping with a sling on is recommended after shoulder surgery to immobilize your shoulder and to prevent any movement which could damage the tissues that are healing.
Sleeping with a sling on after dislocating your shoulder is also recommended to keep your shoulder joint stable and prevent any redislocation of the shoulder.
And sleeping with a sling on is also recommended for a broken elbow or broken arm to support the injured limb and prevent further damage from occurring.
However it's not always required to sleep with a sling on such as for minor injuries or after a couple weeks of healing.
If the injury is minor and not severe you may be able to sleep without the sling and remove it at night and allow for some movement.
And after the initial healing period has passed, you should be able to gradually reduce the amount of time that you wear the sling.
If you do need to sleep with the sling on you should make sure that the sling is not too tight and use a pillow for supporting the injured arm and prevent pressure sores from occurring.
Also if you have any discomfort or difficulty sleeping with the sling on you should let your doctor know.
If you do need to sleep with the sling on the sling should be kept on for the first 6 weeks while in bed and you might find it more comfortable to sleep on your back initially and use a pillow under operated arm for support.
And you may find it more comfortable to sleep in a semi sitting position.