Science

It's reference 13 in the full paper, which is here: Kendall-Bar, Jessica M.; Williams, Terrie M.; Mukherji, Ritika; Lozano, Daniel A.; Pitman, Julie K.; Holser, Rachel R.; Keates, Theresa; Beltran, Roxanne S.; Robinson, Patrick W.; Crocker, Daniel E.; Adachi, Taiki; Lyamin, Oleg I.; Vyssotski, Alexei L.; Costa, Daniel P.; Brain activity of diving seals reveals short sleep cycles at depth, Science, 260-265, 380, 6642, (2023)

Reference 13:

M. J. Weise, D. P. Costa, Total body oxygen stores and physiological diving capacity of California sea lions as a function of sex and age. J. Exp. Biol. 210, 278–289 (2007).

The latter paper is fully open sourced, but basically the idea is that phocids (true seals), the order to which Elephant Seals (Mirounga angustirostris) belong, store significant quantities of their oxygen in their muscle tissue, in myoglobulin, as well as in hemoglobulin in their blood. Their ability to do so apparently changes with age.

In sleep, they are in a reduced metabolic state, probably with lower body temperatures.