Brain Tour : Our Brain : The Principal Part of the Brain :

The diencephalon (dia=through; enkephalos=brain) consists principally of the thalamus and the hypothalamus.

Thalamus

The thalamus (thalamos=inner chamber) is located at the base of the forebrain . It is a paired structure, with half on either side of a central fluid filled cavity.

It consists of densely packed clusters of cell bodies which evaluates incoming sensory signals and passes them to appropriate areas of the cortex. The thalamus has been rather poetically called the brain's "great relay station."

In addition, the thalamus has the special task of relaying impulses to the cortex that help maintain consciousness. For this, the thalamus relies on the reticular system.It is a region composed of neurons which lie in the thalamus. What does it do , then? For one, all incoming and outgoing communication has to pass through the reticular system. It seems that it is an arousal system that serves to activate the appropriate parts of the brain upon receiving the stimulus. The more messages it intercepts, the more a specific part of the brain is aroused.

When we are sleeping, the reticular system receives fewer messages, and the brain is allowed to relax and initiate the processes associated with sleep. This is why we find it easier to fall asleep in quiet surroundings rather than in noisy places.

Related links
Brain Tour : Our Brain : Looking at the Cerebrum : The Emotional Brain
The Hypothalamus and the thalamus are part of the "emotional brain" - the limbic system. Read about how things get steamy!