As Rabbi Shimon ben Lakish said: The Holy One gives a person an additional soul on Shabbat eve, and at the conclusion of Shabbat removes it from her, as it is stated: “G-d ceased from work and was refreshed [vayinafash]” (Exodus 31:17). Rabbi Shimon ben Lakish expounds the verse as follows: Since G-d ceased from work, and now Shabbat has concluded and her additional soul is removed from her, woe [vai] for the additional soul [nefesh] that is lost.
(יז) בֵּינִ֗י וּבֵין֙ בְּנֵ֣י יִשְׂרָאֵ֔ל א֥וֹת הִ֖וא לְעֹלָ֑ם כִּי־שֵׁ֣שֶׁת יָמִ֗ים עָשָׂ֤ה ה' אֶת־הַשָּׁמַ֣יִם וְאֶת־הָאָ֔רֶץ וּבַיּוֹם֙ הַשְּׁבִיעִ֔י שָׁבַ֖ת וַיִּנָּפַֽשׁ׃ (ס)
(17) it shall be a sign for all time between Me and the people of Israel. For in six days the LORD made heaven and earth, and on the seventh day G-d ceased from work and was refreshed.
Rashi (France, 11th Century): An expanded "heart" for rest and joy, open to comfort, to eat and drink without the soul bothering her.
Rabbeinu Bahya (Spain, 13th Century): During the six days of the week the soul may be compared to someone who is merely a guest without a home. On the Sabbath the soul is like a guest who has found a home. This is why the Torah used the word ויברך ה' את יום השביעי, “God conferred a blessing on that day,” i.e. the soul derives a blessing from its source on that day.
Rabbi Arthur Green: If I have spent a Shabbat at your congregation, you have heard me share my interpretation of Lecha Dodi. I call it a flirtation song with the neshama yeteirah. I do not believe, you see, that the “extra soul” we have on Shabbat comes floating down from heaven at 3:42 in the winter season and or 7:29 in the middle of summer (at least in our Boston climate!). I believe that the soul, the most intimate, and therefore most vulnerable parts of ourselves, is there within us all week long. But it is afraid to come out. It fears being trampled by the pace at which we live, shouted down by the loudness of our encounters with the hustle-and-bustle of ordinary life...But, on erev Shabbat we promise it, “It’s all right, you can come out now. I promise, for the next 24 hours, to live at a slower pace. No rushing, no fighting, no screaming. No despair over the stock market or the business cycle. I promise not to get depressed by watching politicians on television. It’s safe in my Shabbat world; you can come out now.” (CCAR Journal, Personal Theology)