Early riser- changing behaviour

[TerrifiedMothertobe]

Our son has for the last 18 months gone through cycles of early rising. As the days get longer he wakes at 5am, even though his room is dark. The last couple of weeks he has started to sleep longer, not every morning but more often than not. This morning he sleep until 7am for first time in almost a year. Nothing I can put my finger on, aside from the days drawing in.

Is it actually possible his body is now producing more melatonin as the days shorten?

I hope to god it continues, I don't feel knackered tonight for same time in ages.

Why why why?!

No advice, but hand holding here.
DS1 has always been an early riser.
The saving grace for us was a clock that changed color when he could get up (it looks a bit more grown up than a groclock, and is easier to use, but same idea - DS2 has a grow clock).
It is set for 6.20 (the time we get up in the week), but he usually gets up at 5.10-5.20, goes to the bathroom, and then waits in his bedroom (he chooses some toys to take up, and changes them occasionally) til the clock goes green. I hear him get up, but then doze, undisturbed, til our alarm goes off at 6.15. Weekends are a bit of a [zzzzzzzz] but since he's already been awake on his own for an hour, I don't think its fair to shift it to something like 7am so we get a lie in!
We got it when he was 3.5, I think, and its been brill. Wish we'd done it earlier.

We tried everything and nothing worked- 445/5/515 was the norm. But the last 10 days he's been up at 6am or later more often than not. Bizarre.

I hope I'm not cursing it!