While the needles are in they feel comfortable. They are very thin (only about three times the thickness of a human hair and much finer than the hypodermic needles used to give injections) and are designed to enter the skin with little resistance. Once the needles are inserted (generally from 10 to 30 are used depending on the condition), the needles can be manipulated or electrically stimulated to increase the energy flow. The needles may be left in for 15 to 40 minutes, depending on the ailment. Different people experience different sensations from Acupuncture. Some describe a tingling, pins-and-needles, or a heaviness feeling, others may feel numbness or nothing at all. Most find the sessions relaxing, and many fall asleep during or immediately after treatment.  In general, visits occur once or twice a week for one to two months until therapeutic results are achieved.