Muslims in Southeast Asia and much of the Middle East, including Iraq and Saudi Arabia, are starting the fasting month of Ramadan on Monday.

Millions more, however, in India, Pakistan and Iran, will likely be marking the start of the lunar month on Tuesday based on moon sightings.

Across the world, Muslims fast from dawn to dusk each day for the entire month of Ramadan, abstaining from food and drink and engaging in intense nightly worship, in addition to five daily prayers. For those fasting, that means around 15 hours without food, water, cigarettes or any caffeine.

Ramadan fasting is aimed at drawing worshippers closer to God through self-control, remembrance and humility. It is also a festive time where families and friends gather for evening meals known as "iftar."