Any ideas

So starting dec 1st i started sleep training my 8 month old. On the 8th night he finally slept through the night without waking up. Couple times he would be a little unsettled but still was sleeping. He would sleep 7pm to 6am, but the past two nights he’s been waking at 5am and won’t go back to sleep no matter how much crying. Any ideas why this is happening or how i can get him back to sleep till at least 6? If he’s up at 5 his whole nap schedule gets pushed up and his bed time would be super early !!