All this shows that the condition of the breath changes the mental condition of that moment. Immediately after concentration upon work the rhythm of the breath is very rapid and exercises its influence on the mind. After a while this rhythm changes and becomes normal; so the condition of the mind changes also. In sleep some people work up the rhythm of breath and the condition of mind becomes unsettled and disturbed, but as a rule sleep helps the breath to be normal and its influence produces tranquillity of mind.