My LO drink 120ml every 4 hours of interval but recently keep sleep more and drink lesser.

My LO tends to sleep more.. And for long hours during night from 10pm to morning 6am. However. I tend to feed him once in the middle. He drinks all. But when comes to next morning feed he will left like 50 to 70ml... And then the following feed after 4 hours... He tends to leftover 20ml etc. But he totally looks fine.. I am not sure because my mil taking care of him.. He didnt use to be like this before i start work. His interval was every 3 hours... 120ml which was okay. But the whole timing is in a mess and intake dropping. However his weight is 6.8kg though.