the baby produces CCK (cholecystokinin), both from suckling and from the fatty hindmilk reaching the baby’s stomach at the end of a feed. The Australian Breastfeeding Association explains:
There are actually two CCK peaks, one at the end of a feed, and the other higher peak between 30 and 60 minutes after the feed. The baby sucks, gets sleepy, dozes off for a while then wakes again for a top-up feed. That higher-fat feed causes the second peak and the baby goes into deeper sleep. Top-up feeds are also great for the mother’s milk supply.