Doing something, like writing, every day sounds like a quite regular interval. But in theory that interval could be anything from few minutes up to almost 48 hours. And basically, you could do those both intervals straight after each other and still keep up with the "every day".
In practice " every day" is basically doing something on each consecutive period of time while being awake. So the days are divided not necessarily with any particular times but instead doing something, in this case sleeping. And to be more precise sleeping a proper nights sleep (despite the time you do it) and not counting short naps.
By this definition, I did not write yesterday. I did write twice the day before and by all other measures, I'm still on my streak it still feels a bit strange. I spend a whole period of being awake without writing.
Another way of looking at this "every day" is just simply cutting the day at midnight. But I haven't kept my streak by that rule either. There have been multiple times when I have written the days post after midnight.
It's hard to define such absolute rules. And as long as my post count stays equal to the days passed I think I can still safely say I'm on a streak.