There are a few causes of this ulnar nerve problem. These include:
Pressure. Because the nerve runs through that “funny bone” groove and has little padding over it, direct pressure (like leaning your arm on an arm rest) can compress the nerve, causing your arm and hand—especially the ring and small fingers—to “fall asleep.”
Stretch. Keeping the elbow bent for a long time can stretch the nerve behind the elbow. This usually happens during sleep.
Anatomy. Sometimes, the ulnar nerve does not stay in its place and snaps back and forth over a bony bump as the elbow is moved. Repetitive snapping can irritate the nerve. Sometimes, the soft tissues over the nerve become thicker or there is an “extra” muscle over the nerve that can keep the nerve from working correctly.