The Zookeeper's Wife, by Diane Ackerman

Antonina's brain refused to accept the sad new reality, as a funereal silence hushed the grounds and she tried telling herself that "it wasn't a death sleep but hibernation," the lull of bats and polar bears, after which they would wake refreshed in spring-time, stretch their scruffy limbs, and search for food and mates. It was only a rest cure during the icequake and frostbite days of winter when food hid and it was better to sleep in one's burrow, warmed by a storehouse of summer fat. Hibernation time wasn't only for sleep, it was also when bears typically gave birth to cubs they suckled and nuzzled until spring, a time of ripeness. Antonina wondered if humans might use the same metaphor and picture the war days as "a sort of hibernation of the spirit, when ideas, knowledge, science, enthusiasm for work, understanding, and love—all accumulate inside, [where] nobody can take them from us."