What's the earliest, healthy time a human can get up in the morning?

I know a girl my age who has a job where her working hours are quite flexible, and she has told me that she sometimes arrives to work as early as 5 AM in the morning - which of course means that she gets out of bed even earlier than that, probably around 4 AM or something like that.
And I was wondering - just how early can humans get out of bed in the morning without getting up "too early"?
For example, is it unhealthy to go to sleep at 7 PM and start your day at 3 AM?
For some reason I have the feeling that 4 AM is the limit, but I am not sure about that.

The reason I am wondering this is because even though I am a major night-owl, I do also really like to be up very early in the mornings when I actually get to bed reasonably early, and I was wondering how early it is reasonable to get up.

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And I was wondering - just how early can humans get out of bed in the morning without getting up "too early"?

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Nowadays, it isn't so much about "when to wake up" as it is about "how much quality sleep should you get". There are ways to influence your circadian clock, and even combat effects like the jet lag, and while solar light can have a pretty important effect on the regulation of our sleep, you can still lead a healthy sleep life even with those kinds of shifts (I also work in both morning/afternoon/night shifts).

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For example, is it unhealthy to go to sleep at 7 PM and start your day at 3 AM?
For some reason I have the feeling that 4 AM is the limit, but I am not sure about that.

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Why would 4am be the limit? The only reason as to why we should worry about it is because of exposure to light, which considering due the technology era which we live in, makes matters a bit more complicated (and has negative and positive consequences). The reason for going to bed and waking up at "normal" times is simply because our clocks are "designed" that way: our ancestors evolved as daily creatures that slept when there was a lack of light, which favored our lack of nocturnal traits. If we were all bats, we probably would think that sleep throughout the day to be the most natural.

That said, factors like the seasons can influence your circadian clock, which is why you should push your sleep pattern around 15 minutes forward or backwards depending on the start of exposure to natural light. I can't link it at the moment, but there are studies that show the differences between natural light and artificial light as signals that tell your body that is time to wake up (in short, when you want to wake up, let the sun light fill the room, artificial lightning is not effective at all). If you search around, there's also reports of specific hours in which you "should" be waking up (just like melatonin production peaks at specific hours), but even then, sleep quality and quantity tend to be more important if waking up early would imply sleeping only a couple of hours the majority of the week.

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The reason I am wondering this is because even though I am a major night-owl, I do also really like to be up very early in the mornings when I actually get to bed reasonably early, and I was wondering how early it is reasonable to get up.

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Find a sleep pattern your comfortable with, and stick to it: that's the true recipe for healthy sleep: consistency ;)

Thank you. :D

The sunlight hours are sort of tricky in my country, since they vary a lot.
Right now I study in the northern parts of Sweden, and in the summers it is daylight almost all the time and then darkness almost all the time during the winters (heck, I remember being awake at midnight during high summer once and I could almost see the sun!);
so that makes it a bit tricky to keep consistent sleeping habits.
But I will try getting used to waking up at 4 AM and see how I like it - at the very least, I am going to get used to get up well before 8 AM.

For me, I think, there's no really definite time for us to consider in waking up especially in the morning. The most important thing to remember is to have an average of 7.5 to 8 hours of sleep per night.