Younger babies tend to not sleep by themselves. You have to help them to sleep by rocking, patting or hugging them. It will be better to help them to sleep than to let them stay awake for too long. By the time they are overly tired, they will be fussing a lot and it will be harder to let them sleep then.
When my baby is at that age, he requires a lot of touch from me so I hugged him to sleep. I got myself into a comfortable position and let him sleep on me. Be careful not to fall asleep! It will risk flattening them.