Actaully, there is scientific evidence of that very thing - as has already been discussed. Evaluation of critical flicker fusion frequency and perceived fatigue in divers after air and enriched air nitrox diving. - PubMed - NCBICurrent thinking is that oxygen is as narcotic as nitrogen or even more so. Nitrox does not reduce narcosis. If you feel less tired, congratulations. The placebo effect works for you. There is no scientific evidence that backs up your claim.
RESULTS:
The two groups were comparable in sex ratio, age and diving experience. The change in perceived fatigue level after a single dive was significantly lower when EANx was breathed compared to air dives (VAS; P < 0.001). Compared to pre-dive, CFFF decreased by 6% in the air group (P < 0.01) but increased by 4% in the EANx group (P < 0.05). The post-dive difference between the two groups was highly significant (P < 0.001).