Learn Russian: Зимами все медведи впадают в спячку. - In winters, all bears fall into hibernation.

The bare instrumental plural Зимами specifically expresses temporal iteration, meaning the action happens repeatedly over multiple winters (every winter). While the singular Зимой (in winter) could also work to state a general fact, Зимами strongly highlights the recurring seasonal cycle.

Why does спячка take the accusative case (в спячку) here?

The verb впадать means to fall into. Because the bears are entering a new state, Russian treats this like a change of direction or physical motion. Just like walking into a building, entering a state uses the preposition в followed by the accusative case (спячку), rather than the prepositional case.

How does the word Зимами dictate the verb aspect of впадают?

Because Зимами establishes that the action is a recurring, habitual event (happening every winter), the verb must be in the imperfective aspect (впадают). A perfective verb would imply a one-time completed result, which contradicts the repeated timeframe.

Is впадать в спячку just for animals, or can it be used for people?

Впадать в спячку is the literal, scientific phrase for an animal going into hibernation. However, it is also widely used in casual speech to describe a person who becomes extremely lethargic, sleepy, or inactive during the cold months.