We studied sleep using non-invasive EEG […] Surprisingly, slow-wave activity decreased not only during NREM sleep but also during rumination. […] Reindeer spent less time in NREM sleep the more they ruminated. These results suggest that they can sleep during rumination. The ability to reduce sleep need during rumination—undisturbed phases for both sleep recovery and digestion—might allow for near-constant feeding in the arctic summer.
https://www.cell.com/current-biology/fulltext/S0960-9822(23)...
—and the key quote from the abstract:
We studied sleep using non-invasive EEG […] Surprisingly, slow-wave activity decreased not only during NREM sleep but also during rumination. […] Reindeer spent less time in NREM sleep the more they ruminated. These results suggest that they can sleep during rumination. The ability to reduce sleep need during rumination—undisturbed phases for both sleep recovery and digestion—might allow for near-constant feeding in the arctic summer.