Shabbath Shalom

Spiritual highs are attainable only when one frees oneself of such human concerns as pain, anguish, anger, etc. The kind of sanctity Yaacov strove for needed to be based on the attainment of a degree of serenity in his earthly life, something that we nowadays experience only on the Shabbat, a day that G-d has imbued with the quality of rest.
Whenever Yaacov experienced the kind of spiritual serenity we are meant to experience every Shabbat, he qualified for the name "Israel". Whenever Yaacov experienced worries, etc. this serenity departed from him similar to the departure of the additional soul from every Jew at the end of the Shabbat. At such times the Torah reverts to referring to our patriarch as being merely "Yaacov"