Now two things devour my life,” wrote the poet William Butler Yeats. “The things that most of all I hate:/Fasting and prayers.” This week, the world’s billion Muslims and 12 million Jews will be fasting and praying in honor of Ramadan and Yom Kippur. Fasting is common to nearly every major religion; mystics fast to induce divine visions, and the rest of us fast to remind ourselves periodically that worldly pleasure is fleeting. Fasting is, in one respect, an exercise in discipline. “It’s analogous to taking a vow of celibacy,” says the atheist Sam Harris, who is interested in meditation. “It’s not so much the direct effect of not having sex that is being sought, necessarily, but the freedom from all the related entanglements, hopes, cravings, etc.”