The brain stem has been shown to be involved in the some of the many factors that create sleep. It has been shown to create chemicals that are crucial to achieve sleep. In other words, these chemicals help brain cells talk to each other during sleep. During sleep brain cells are not resting, but transmitting information to each other via neurotransmitters. The chemicals help transmit and regulate this information.
Serotonin moves up to the brain and down to the spinal cord.
Being a neurotransmitter, it is believed that serotonin helps brain cells communicate with each other. Of the chemicals listed above, serotonin is the one many physicians believe causes sleep. When Microsoft is quoted, I am referring to Microsoft Encarta 97.