Why would it be very light out at 3 AM? My brother noticed something very odd last night: it was amazingly light out when he stepped outside for a smoke even though it was between 3 AM and 3:30 AM EDST and we are near Toronto Ontario Canada, not in the Far North. He is outside at that time most nights and is no stranger to being up very very early at all times of the year. He has always lived here and is 63 years old, not an inexperienced child. He said the amount of light in the sky was equivalent to the amount of light you'd see in the mid-afternoon of an overcast day. He'd never seen anything like it.
Since I have a lifelong love of science fiction, he asked me if I had any idea why this might have been; he thought there might be some natural phenomenon that might explain it. I asked if he had used some kind of intoxicant but he assured me he was completely sober.
I'm at a loss to think of any plausible source for his experience but I'm not a scientist of either the professional or amateur kind so I thought I'd ask here if there is some rare atmospheric condition that might explain it.
I come from a similar latitude and enjoyed the night skies and two things that can add to &quot;the amount of light in the sky&quot; that I can think of. The first is the near-full moon as mentioned above, and the second is unusually low clouds. Being near Toronto and outlying communities, there is a great deal of light produced, and some goes up. On nights when the clouds or even semi-transparent haze are present and unusually low, they can reflect light pollution more often than normal. Add to that the light from the Moon and it should be enough.
The reason I mention latitude is that when the ground is covered in white, reflective snow, the light pollution is even worse; lights projected preferentially downward are more strongly reflected back upwards. I don't know if there's any snow on the ground still there; I haven't seen snow in way too many years.
Since you didn't mention that your brother noted the full Moon, I'm guessing it was cloudy and so they didn't see it. So I'm guessing that instead they saw clouds or haze lit by it and/or by local light pollution.