Sleep is primarily a function of the brain. So it’s expected that without a brain that’s developed enough to actually Sleep, the animal won’t Sleep.
Example of some of the animals which seems to be doesn’t sleep at all:
- Cnidarians(Jellyfish, hydra, coral) never Sleep
2. Sponges, plants, bacteria and fungi also don’t Sleep
3. Arthropods like crustaceans, arachnids and insects don’t sleep as well but they undergo a metabolic slowdown state called as Torpor.