What causes numbness in the hands that happened overnight?

Nocturnal numbness: The most common cause of numbness of the hands occurring at night is carpal tunnel syndrome. This occurs because we usually sleep with our wrists curled up or flexed, and this puts a lot of pressure on the median nerve in the wrist. The symptoms usually only involve the thumb and next 2 fingers, maybe part of the 4th, but not the 5th. Other cause is pressure on a nerve in neck, can affect all 5.

Carpal Tunnel: I assume you mean numbness in the hands that happens overnight and not happened last night. The most common cause of numbness in the hands that happens overnight is carpal tunnel syndrome. This is compression of the median nerve in the wrist. The easiest remedy for this complaint is using cock-up wrist splints that can be purchased in any drug store. This would be a good start.

Nerve compression: Nerve compression, whether in the neck (pinched nerve or herniated disk) or in the arm ( carpal or cubical syndrome), can cause numbness or tingling in the arm. Peripheral nerve compression like carpal and cubical tunnel syndrome, typically produce symptoms in the hand/fingers. Compression in the neck can cause symptoms throught the arm. Carpal tunnel syndrome is the most likely cause.