To maintain the health and longevity of your garments, organic wool is best washed by hand.  Many modern woolen items are treated with chemicals to allow them to tolerate machine-washing.  Certified organic wool does not contain such chemicals.  Some washing machines have a special wool-cycle, which maintains a lukewarm water temperature throughout the wash and rinse cycle, as well as minimizing agitation of the garment, which can lead to felting / shrinking.  However, wool does not need to be washed each time it is worn - simply air out overnight and they are fresh in the morning.  Can be worn for extended periods without need for washing. 

 See Caring for Wool.