Acknowledge the slumber of the vessel. There is better work done consistently in a 3-hour block over a week, than a forced 5 hour block each day. One reaches the point of minimal returns on the same effort. Taking the time and coming back the next day for the next block will aide you in building up to doing larger blocks of deep work, as also ensure a consistency of energy and intention within the output of those blocks of work. If 3 hours is pushing it, dial back to where you finish up the block still feeling fresh, and then edge your way up to larger blocks over the course of the week, month, and/or year. Chronic tiredness ensues when one pays less attention to the sleep and down-time needs of the body.
Let go, and enjoy the process. That process involves sleep too.