Instead of sleeping, Lex chose to cultivate to recover. He needed to be in top shape to protect himself while the rest of his cohorts fought the Druks or any other enemies that might crop up.
A benefit of all the exertion he was experiencing was that his cultivation was progressing quickly. While the concentration of spiritual energy mattered little for Qi training, since he was absorbing individual strands of Qi, the indirect benefits he was receiving of being in an energy rich environment, such as eating healthier food or recovering from fatigue quicker, were speeding up his cultivation nonetheless.
