To be honest, we're not sure. There have been many attempts at explaining why this happens and why acupuncture works in general. Our sense is, the presence of the needles causes our central nervous system to move into a clear pattern of rest (parasympathetic), allowing for our quickest healing and recovery to take place. Not unlike when we sleep at night.
This may explain why acupuncture is so effective at helping people overcome the many troubles associated with high stress levels – a state we can find ourselves in which is characterized by our nervous systems staying in a “fight or flight mode' (sympathetic) for extended periods of time.
Remaining in this state for long periods of time can keep us from recovering in an ideal way, leading to nagging injuries, sleeplessness or illness.
We can tell you, helping people get into a sleepy state is one of the most predictable and best effects acupuncture has to offer.