Young babies also tend to get sleepy after certain activities: for example, they might get tired after every time they are breastfed. (momtricks.com)
I don't supervise the older grades at lunchtime, so I can't speak for their older boy, but the younger one is known for getting sleepy in the afternoons. (easylunchboxes.com)
That's why you don't get sleepy when you're on your tablet or watching TV. (parsleyhealth.com)