Sometimes the explanation is simple: you slept on your arm, bent your wrist for too long, or put pressure on a nerve.
But recurrent or persistent numbness deserves more attention.
Possible causes include carpal tunnel syndrome, ulnar nerve compression, a pinched nerve in the neck, diabetes, vitamin B12 deficiency, medication effects, pregnancy-related swelling, repetitive strain, and other neurological or metabolic conditions.
The pattern of numbness can provide useful clues.
If the thumb, index, and middle fingers are affected, the median nerve may be involved.
If the ring and little fingers are affected, the ulnar nerve may be involved.
If numbness travels from the neck through the shoulder and arm, a cervical nerve problem may be possible.
And if numbness appears suddenly along with facial drooping, speech difficulty, severe weakness, vision changes, loss of balance, or other neurological symptoms, treat it as an emergency.
Likewise, sudden hand or arm numbness accompanied by chest pressure, shortness of breath, sweating, nausea, or fainting requires urgent medical attention.
