Abraham Harold Maslow was an American theorist who studied the needs of humans and how satisfying these needs can motivate people to work harder and be happy. He started his studies observing the behaviour of monkeys. Whilst researching the monkeys he realised that some needs took priority over others. He took these ideas and produced his famous hierarchy of needs.
At the bottom of this hierarchy were the physiological needs of the person. These needs included water, oxygen, heat, vitamin C, rest, avoiding pain and having sex. The lack of these physiological needs leads to the body craving for things that have satisfied these needs before.