It's a question facing a small but growing number of Muslims celebrating the holy month of Ramadan on the northern tip of Europe, where the the sun barely dips below the horizon at this time of year.
In Rovaniemi, a northern Finland town that straddles the Arctic Circle at 66 degrees north, the sun rises around 3:20 a.m. and sets about 11:20pm. That means Muslims who observe Ramadan could be required to go without food or drink for 20 hours.
In a few years, Ramadan will begin even closer to the summer solstice in late June, when the sun doesn't set at all.
"We have to use common sense," said Mahmoud Said, 27, who came to Finnish Lapland from Kenya three years ago.
To Said, that means following the fasting hours of the nearest Muslim country: Turkey.
"It involves 14 or 15 hours of fasting which is okay, it's not bad," said Said, who works for a non-governmental organization helping immigrants settle in the area. He estimates there are a little over 100 Muslims in Rovaniemi, mainly from Iraq, Somalia and Afghanistan.
There is no unanimity on how to deal with the issue, which is becoming more pressing as more Muslim immigrants find their way to sparsely inhabited areas near the Arctic.
In Alaska, the Islamic Community Center of Anchorage, "after consultation with scholars," advises Muslims to follow the fasting hours of Mecca, Islam's holiest city.
The Dublin-based European Council for Fatwa and Research, however, said Muslims need to follow the local sunrise and sunset, even up north.
"The debate on how to do this in the north has been on going on for a few years," said Omar Mustafa, the chairman of the Islamic Association of Sweden. "We fast according to the sun. As long as it is possible to tell dusk from dawn. This applies to 90 per cent of Sweden's Muslims."
The few Muslims who live so far north that they are awash in 24-hour daylight should follow the daylight hours the closest city in Sweden where you can tell dawn from dusk, he said, noting that it's permitted to break the fast for health reasons.
Kaltouma Abakar and her extended family of nine relatives came to Finland from Sudan's Darfur region four years ago. She opts to observe the local Lapland sunrise and sunset times before breaking the fast in her downtown Rovaniemi apartment.
Kaltouma explains that she gets up early and works until the afternoon, then starts cooking the family's iftar meal around 5 p.m.
"The time of Ramadan fasting is very long, and breaking the fast can be around 11:30 in the evening. The time you're supposed to eat your breakfast is 2 o'clock in the morning," the 31-year old said.
In the kitchen, Kaltouma's two daughters — aged 11 and 6 — help prepare the food. They fry chicken and pastries filled with tuna in scalding hot oil. A pot of rice simmers on the stove while one girl kneads cornmeal dough which they'll dip into a chicken broth and eat with their fingers — traditional Sudanese style — a few hours later.
Apart from the late sunset times, Kaltouma said the lack of "Muslim food" locally in Rovaniemi can be a challenge. She sometimes has to wait several days for halal meat and other traditional ingredients to come from the larger cities of Oulu, or Helsinki in the south.
Even though, technically, there is nightfall in Rovaniemi at this time of year, there is no true darkness. Instead, there'sdn a grey gloaming with occasional dappled rays of sun reaching over the northern horizon, giving the city a mystical quality even in the supposed dead of night.
The dates of Ramadan change according to the lunar calendar, moving back 11 days each year. That means that by 2015 there will be no sunset for a month when Ramadan falls closer to midsummer.
Still, Kaltouma says "there is going to be at least 10 minutes for us to break the fast."
She said there is one positive aspect of observing long fasting hours in the Arctic during Ramadan: the cool temperatures.
"Unlike Africa, here in Finland you don't get thirsty often. No matter how long you fast, you don't get the urge for water."
Associated Press writer Karl Ritter in Stockholm contributed to this report.