The reticular activating system (RAS) is a part of the mammalian brain located in the brain stem. In human biology, it is believed to play a role in many important functions. These include sleep and waking, behavioral motivation, breathing, and the beating of the heart.

A loose network of neurons and neural fibers running through the brain stem make up the reticular activating system. These neurons connect up with various other parts of the brain. There are two portions to the system, the ascending reticular activating system, and the descending reticular activating system. The ascending RAS connects to the brain parts called the cortex, the thalamus, and the hypothalamus. The descending RAS connects to the cerebellum and to nerves responsible for the various senses.

Functions of the reticular activating system are many and varied. Among other functions, it contributes to the control of sleep, walking, sex, eating, and elimination. Perhaps the most important function of the RAS is its control of consciousness. It is believed to control sleep, wakefulness, and the ability to consciously focus attention on something. In addition, the RAS acts as a filter, dampening down the effect of repeated stimuli such as loud noises, helping to prevent the senses from being overloaded.

In cases of reticular activating system damage, a coma may result. Due to the location of the RAS at the back of the head, this area is particularly prone to being damaged in automobile accidents. Unusual activity of the RAS area of the brain has been linked with sleep disorders such as narcolepsy, with chronic fatigue syndrome, with Alzheimer’s disease, and with Attention Deficit Disorder (ADD).

Other than trauma, there are a number of things that affect the RAS, such as drugs and chemicals. Many psychotropic drugs, which are drugs that act on the brain and the nervous system, are believed to function by directly affecting the RAS. Melatonin also affects this area of the brain, and general anesthetics work by turning off consciousness via the RAS.

The RAS appears to play an important role in dreaming. Scientific observation using brain scans and electronic equipment shows that during deep sleep, the activity in the reticular activating system is much reduced. During rapid eye movement (REM) sleep, however, which is when dreaming occurs, the activity in the reticular activating system increases to levels similar to those that are seen during wakefulness.