Why is it that we fall asleep while meditating on the Word of God and in prayer?? God is not one we just resort to as our last-minute end of the list activity. He is to be the priority and of the utmost importance. Thus, like anything else, there is no way we can be in His presence and make breakthroughs when it is the thing we do when we have the least energy! He is the infinite God, Incomprehensible. To break in, we must be willing to sit at His feet and just listen until He speaks. Waiting requires more energy then doing anything in this world. It requires much strength and power to be able to wait on the Lord.
Why is it that when we want something, we are willing to not sleep and keep ourselves awake, but with the Lord we allow sleep to take over. Being with the Lord, He allows us to sleep when we say we want to meditate on Him. He knows that we are tired, thus we fall into rest. He wants us to rest then when we have the energy we can continue to dig deeper into the attributes of Him. The genius of God, He makes it so that we rest when we meditate on Him when we are tired, and when we are not He waits patiently until his perfect timing releases self-disclosure through the Spirit.
But it is not to say that we should then just allow sleep to take over whenever we fall sleepy when we think about God, but rather take it as a sign that we should focus on God the most when we are full of energy and refreshed and not as our last thing of things to do. We should continuously be in communion with the Spirit whether full of energy or drained. Seek Him as a man, as a friend, as a close partner, and as a Father.