Fuel as well.
I had a bastardised version of Palisade this morning (same work but reduced recoveries/ warm up/ cool down to get to 1:15), and found the first interval hard.
On waking I had a gel, finished a carb mix bottle before first interval, RPE was definitely less for rest of the intervals. Not enough to crank up the intensity, but I really struggled through the first interval, less so as it went on.