Our results show markedly different dependencies on high-laser power for the ODMR contrast in bulk diamond and NDs. While contrast and NV -spin polarization steadily increase with laser power in the bulk diamond, with little to no variation across the sample, they show a more complex and heterogeneous behavior in the NDs. In bare as-deposited NDs we observed a decrease in the ODMR contrast at the highest power, reflecting a more efficient NV -→ NV0 photoconversion compared to the bulk, and implying a reduction in the pool of NV -centers. The non-monotonic behavior in NDs is likely to be determined by the interplay between spin and charge dynamics under continuous laser illumination. NDs showed a high tendency to aggregate, forming much more heterogeneous systems than in the homogeneous bulk diamond. For NDs internalized in cells we observed a qualitatively similar trend as in the bare NDs, with a different, more variable behavior at the highest laser power, suggesting that the cellular environment may have a role in the dynamics of NV charges, perhaps due to the different pH or to surface interactions with different proteins in the cytosol.