[prerok]

We know plants have a diurnal cycle and react to sun/day and some visibly change between night and day. If we say that one of these states is less active, we may decide to call it dormant. Dormant comes from latin dormire, which is sleep. So... why not?

[cubefox]

Animals have a sleep-wake cycle that is usually synchronized with the day-night (24 hour) cycle of the Earth. But this synchronization is not essential. All animals with nervous system have a sleep-wake cycle, even if they live underground or in the deep ocean, where the day-night cycle has no significant effect. So there must be an actual need for the sleep-wake cycle that is independent of the rotation of the Earth.