Bring vitality into the performance of the mitzvot

When Rebbetzin Rivkah (wife of Rabbi Shmuel, the fourth Chabad-Lubavitch Rebbe) was eighteen, she fell ill and the physician ordered her to eat immediately upon awakening. She, however, did not wish to eat before praying, so she woke up even earlier and prayed—so that she could eat breakfast at the time she had been used to waking up beforehand. Understandably, her new schedule, with now reduced sleep time, did not cause her health condition to improve...

When her father-in-law, the Tzemach Tzedek (Rabbi Menachem Mendel, third Chabad-Lubavitch Rebbe) learned of his daughter-in-law's behavior, he said to her:

"A Jew must be healthy and strong. Regarding mitzvot, the Torah says: 'Live in them,' meaning, one should bring vitality into the performance of the mitzvot. To be able to infuse mitzvot with vitality, one must be strong and joyful."

He concluded: "You should not be without food. Better to eat for the sake of praying than to pray for the sake of eating..."

Source: Hayom Yom 10 Sh'vat