What causes split nights with toddlers?

[Seasonofthewitch83]

DD is almost 3 and we have always co-slept. For years she was the most god-awful sleeper - waking hourly, taking hours to fall asleep etc.

In the last few months once she had established dropping the nap, she has FINALLY been sleeping 7-7. She can now go pretty much the whole night sleeping alone. .
The problem is, if she wakes up, she WAKES UP. At least once a week she will wake up around midnight and then is wide awake until at least 4.30am. Her record was 11pm-5am wide awake.

I ignore her, keep it boring etc but after 90 minutes she then starts jumping on me, wailing etc.

She doesn't try to get out of bed, and eventually I give up after a few hours and hand her some soft toys to play with.

What causes it? I rule out things like pain, the temp of the room etc.

She goes to sleep fine, its like she gets 5/6 hours sleep in to recharge and thats enough to power her through for a few hours.

To not drip feed, she is awaiting referral for potential ASD and her understanding is a bit behind.

My DS is just 2 and does this every so often. With a similar wake window in the night as well.
I can sometimes figure it out, but generally I've no idea either!
He won't do it for a couple months then for a few weeks he will then stops again. Development brain activity is the only thing I could put it down to!

Thats what I am wondering, she is going through so many leaps lately that perhaps her brain is just very excited!

Does she still nap? In which case I would cut the nap. I would try a later bedtime as she maybe doesn't need 12 hours. Usually split nights are due to too much day sleep or the last wake window is not long enough. I'd also check she isn't watching anything too overstimlating or scary