Refined Blood, Faint Embarrassment
Under normal circumstances, one day wouldn't seem that long to be training, especially when most high-leveled individuals trained nonstop to perfect whatever techniques they wished to create.
However, Kieran's circumstances were far from normal.
For starters, his environment was unparalleled in terms of creating a suitable environment to train in, with the surrounding green and blue lights creating a serene atmosphere.
Moreover, when they came in contact with his body, Kieran found that he felt rejuvenated, as if the fatigue was forcibly drained away from his body.
Considering that these lights were powerful enough to allow Gaea's spiritual manifestation, it wasn't odd that they could remove Kieran's fatigue, allowing him to train far more efficiently than normal.
In addition to these lights, Nameless also modified the Archaic Verity Compendium to release energy that helped maintain his focus.