Reykjavik has roughly 4 to 5 hours of daylight around the December solstice, and that single number shapes every winter Golden Circle itinerary. The loop itself still fits comfortably inside the daylight window, but the margin for extras shrinks and grows month by month.
November
About 5 to 8 hours of daylight, shrinking through the month. Early starts matter; the three main stops fit, but optional extras need planning rather than improvising.
December
Roughly 4 to 5 hours around the solstice, with sunrise near 11:20 and sunset around 15:30 in the Reykjavik area. This is the tightest window of the year: enough daylight for the three main stops, but little room for improvised extras.
January
About 4.5 to 7 hours, gaining quickly late in the month. Early January feels like December; by month-end the day noticeably opens up.
February
About 7 to 10 hours. A comfortable margin returns for add-on stops and longer walks.
March
About 10 to 13 hours. Daylight stops being a constraint; weather, not light, decides the day.
Short days are not only a limitation. The winter sun stays low even at midday, so the route spends much of the day in long, golden light that photographers usually only get at dawn and dusk. Winter departures concentrate the daylight hours on the three main stops, with the dark hours used for the transfer legs at either end of the day.