I think I know what you mean too
... I've googled baby's sleep cycle and found this:
0-10mins - starting to fall asleep
10-20mins - getting deeper into sleep
20-30mins - deep sleep/heavily asleep
30-40mins - coming out of deep sleep
40-50mins - light sleep/easy to wake up
and then the cycle continues again
So in reality your LO is getting some deep sleep already in the 1st part of sleep cycle but would be better rested if he put himself back to sleep for another sleep cycle...
That's how I understand it anyway