All of us are aware of the many different states of brain activity,including sleep, wakefulness, extreme excitement,and even different levels of mood such as exhilaration, depression, and fear.All these states result from different activating or inhibiting forces generated usually within the brain itself. The human brain is the center of the human and is a highly complex organ. Enclosed in the , it has the same general structure as the brains of other , but is over three times as large as the brain of a typical mammal with an equivalent body size. Most of the expansion comes from the, a convoluted layer of neural tissue that covers the surface of the brain. Especially expanded are the frontal lobes, which are associated with such as self-control, planning, reasoning, and abstract thought. The portion of the brain devoted to vision is also greatly enlarged in human beings. In this site we present brief surveys of specific states of brain activity, beginning with sleep.