Is that your dominant hand? It could be a symptom of carpal tunnel syndrome. Do you have any pain? Have you been doing repetitive motions with your left hand, such as driving a lot, typing, using power tools, etc? Have you had any injuries to your hand or wrist?

If you don't think that is what it is, you should go see your doctor about it if it continues. Numbness in the extremities can be a sign of something else going on, such as a thyroid imbalance or diabetes.