Heimdall is the guardian of the gods. He will blow a horn, called the Gjallarhorn, if Asgard is in danger. His senses are so good that he can hear the grass grow and he can see to the end of the world. Heimdall could hear a leaf fall. He does not need any sleep at all.
Heimdall was said to be the last of the gods to die at Ragnarok when he and Loki would kill one another.