Nature's Insomniacs: Animals That Do Not Sleep

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Dolphins can rest while keeping one half of their brain active. They also sleep with at least one eye open.

Bullfrogs don't have a clear sleep-wake cycle. They enter a state of dormancy, where they are inactive and have a lowered metabolism.

Fruit flies are resistant to sleep deprivation and some can live their entire lives without sleeping.

Upside-down jellyfish don't sleep because they don't have a brain or nervous system.

Some shark species don't sleep because they need to keep moving to breathe.