How do narwhals sleep?

Narwhals sleep by shutting half of their body down at a time. They close one eye and shut down the associated side of the brain. Later, they close the other eye and shut down the other associated side of the brain. They do so when they safely are hidden away in icy crevices from predatory humans [Homo sapiens sapiens], killer whales [Orcinus orca], and polar bears [Ursus maritimus]. They also do so when they safely are grouped into pods of 10-100.