One of the books I'm reading, "Your Baby Week by Week" by Simone Cave and Dr Caroline Fertleman, states in week 6: "As your baby becomes more sociable, he'll start to resist falling asleep because he'd rather stay awake and play. So instead of dropping off to sleep (...), your baby will actually fight sleep because he's excited about his newly discovered world."
As my baby sleep extremely little during the day, I was surprised to read this paragraph as I was suspecting that he's already resisting falling asleep during daytime.