most guys unfortunately do not know when they need a break. only experience and really knowing your body is the way
a lot of people get tired cause they eat a big meal and get sleepy after.. when your body digests food it uses up a lot of its energy
in the end you need to be doing maintenance on your body, just like your car. i recommend bikram yoga once a week, it will decompress your discs/joints and help mobility/flexbility.