Muslims affirm the duty to fast from dawn (fajr) to sunset through the lunar month of Ramadan, abstaining from food, drink, and marital relations during daylight hours. It is the fourth of the Five Pillars, commanded in the Quran, which prescribes the fast for the month in which the Quran was first sent down and frames it as a discipline meant to cultivate taqwa, God-consciousness, rather than mere abstinence (2:183-185). The sick, travelers, the pregnant and nursing, and others are exempted, making up the missed days later or, where that is not possible, feeding the poor in their place.
Because the Islamic calendar is lunar and unadjusted to the solar year, Ramadan cycles slowly through every season over roughly three decades, so believers in different generations fast through short winter days and long, demanding summer ones in turn, a fact the tradition treats as part of the discipline's fairness across a lifetime rather than an accident to be corrected. Classical jurisprudence developed a fairly technical casuistry around what breaks the fast, disputing, for instance, whether an injection, an inhaler, or unintentional swallowing invalidates it, with the different schools reaching different conservative and lenient rulings on each case.
Sufi writers, al-Ghazali among the best known, distinguished the fast of the body from a fast of the tongue, the eyes, and the heart, holding that abstaining from food while still lying, looking with desire, or harboring malice falls short of the fast's real aim; this interior reading does not replace the physical requirement but is presented as its completion.