Due to the current Covid-19 pandemic, all regular services and hours may be subject to change. Thus, if the idea is to spend the night in the public area, consider a plan B in case the stay inside the airport is only allowed for those who are close to boarding.
Spending the night at the airport
Keflavík airport is small and opens around the clock. Although staying inside the enclosure during the night is discouraged (you will even see many warning signs inside the terminal), the airport staff generally won't bother sleeping passengers, although they will wake everyone up at around 4:00 am. The place, although quiet at night, is pretty cold and bright, so it is recommended to have a sleeping bag, blankets and a mask.
Where to rest: You can find some benches to lay down (especially next to the C gates), but these are scarce, so your only option will probably be the floor. Depending on your airline and flight class, you could also enter the airport's VIP lounge, although this closes at night. On the other hand, there is a hotel 100 meters away from the terminal.
Baggage: Between April and September there is a left-luggage service that operates 24 hours a day. At other times of the year it closes at night.
Eating and drinking: Eateries only operate during flight hours.
Restrooms: There are showers inside the VIP lounge, but not inside the public toilets.
Wi-Fi: Free and unlimited.
Other services available around the clock: ATMs, children's play areas, baby changing facilities.
The Airport Hotel Aurora Star is found 100 meters away from the terminal. There are more options nearby, some of them with transfer services for their guests.
Home Guesthouse (2.3 km/1.4 miles)
B&B Guesthouse (3.1 km/1.9 miles)
Little White Castle (3.1 km/1.9 miles)
Rose Guesthouse (3.2 km/2 miles)
Nupan Deluxe (3.3 km/2.1 miles)