While most animals do sleep or exhibit sleep-like behaviors, there are a few exceptions: certain reptiles like caiman, saltwater crocodiles, and green iguanas display vigilance while at rest; and while birds mostly sleep, some may do one-half of their brain at a time to stay alert if there's danger. Dolphins and sperm whales may only shut down portions of their brain at any given time to preserve breathing; other whales and many seals sleep deeply underwater for short spans.