Lets have basic understanding on fingers during mudra. Every finger of hands relates to one of the five element of the nature. The thumb relates to fire, the index finger to air (matter in gaseous form), the middle finger to ether (space), the ring finger to earth (solid matter), and the little finger to water (liquids) and the palm of the hand relates to the mind.
When mudras are formed by using thumb (fire element) to lightly stimulate one or more of the other elements as you touch the tip, nail, knuckle or base of a finger. So, we can
- stabilize the influence of the linked element by joining the tip of corresponding finger,
- decrease the influence of the linked element by touching the back of the corresponding finger (nail or knuckle)
or
- increase the influence of the linked element by bring thumb to the base of the corresponding finger
Vata is formed with air and ether and these correspond to index finger and middle finger. And the remaining fingers remain straight.
So, we are concerned with index finger and middle finger only.

The position of fingers and thumb may be interpreted from the above principle on how an element is decreased, increased or stabilized. Mudra illustrated in this figure is Vata Nashak Mudra which is meant to decrease the vata or to soothe the overactive vata dosha. When vata is decreased, it surely helps slows the movement not only physical but mental also and therefore improves the sleep.