Ramadan began on Wednesday evening for 1.6bn Muslims around the world, who will have to fast during daylight hours.
The holy month is in high summer this year and for Muslims in the Northern Hemisphere, abstaining from food and drink is that much more gruelling.
Especially when you get really far north.
In the Icelandic capital of Reykjavik, for example, there will be 21 hours of daylight on Thursday - the first day of fasting.
Sunrise was at 2.55am, and sunset at two minutes past midnight.
Muslims in Anchorage, Alaska, must fast for 19 hours, and in Helsinki for 18 hours. In wintry Buenos Aires, by contrast, there will be just nine hours of daylight.
Or do they?
Observant Muslims must fast during Ramadan because it is one of the five pillars of Islam, according to the Quran. Liquids, sex and cigarettes are also off-limits.
But there are exeptions. The elderly, sick, or pregnant can usually get away with eating and drinking on medical grounds.
And those in the far north may fast for more a "moderate" length of time - 12, 14 or 16 hours, say - if they need to, according to a fatwa published by Sheikh Dr Usama Hasan, a British Islamic jurist who works for the Quilliam Foundation counter-extremism think-tank.
Other scholars disagree, however. Imam Khalid Latif, the Executive Director at the Islamic Centre at New York University, told the Quartz website that "to have a broad-based assumption that longer fasting days should be shortened is problematic."
Some fatwas suggest fasting for the daylight hours of Mecca - around 13.5 hours.
For those struggling to abstain, Google has launched the My Ramadan Companion site, complete with sunrise and sunset times, and entertainment and recipe for suggestions for sundown.