The Neshama Yetera

Rebbe Shimon ben Lakish said: Every Erev Shabbat, Hashem gives every person an extra soul, and on Erev Shabbat, He takes it back.  As it says, “Shabbat vayinafash,” (Shmot 31:17) meaning, “After Shabbat, oy, the extra soul (nefesh) is gone.”

(Betza 16a)