How Parts of the Brain May Sleep in Shifts

Mixed States of Consciousness May Lead to Sleep Behaviors Called Parasomnias

Sleep waves illustrated from the brain. Michel Tcherevkoff/The Image Bank/Getty Images

It is bizarre but true: parts of your brain may be actively awake while other parts are asleep. Migrating animals like birds and fish may actually sleep with half of their brains at a time. How does this work and how might it lead to sleep behaviors? Learn how your brain may sleep in shifts and the potential consequences of conditions called parasomnias.

The Nature of Sleep

First, it is important to understand what is meant by the term “sleep”.

Sleep is defined as a transient and reversible lack of responsiveness to the external environment. It has important characteristics that define the state. Moreover, it can be divided into distinct stages based on observation of the electrical activity of the brain as measured by an EEG.

Let’s imagine that we repeat a classic experiment to clearly identify when sleep occurs. In order to do this, the subjects are seated comfortably with their eyelids propped open. (Though the eyes are typically closed in sleep to prevent dryness without blinking, sleep does not require this to be so.) Above the subjects, a light bulb flashes. With each flicker of the bulb, the subjects are requested to push down on a buzzer. This demonstrates they are conscious (awake) and responsive to their environment.

When the subject first slips into sleep, the light will flash and no buzzer response will follow. The subject has fallen asleep.

However, if we wake these subjects shortly thereafter from the lightest stage of sleep (called stage 1), about half would say they weren’t asleep at all. They would reply with an excuse, that they hadn’t been paying attention, but that they didn’t feel they were asleep. This reveals the paradoxical nature of light sleep: we may perceive it as wakefulness, even though we are unresponsive to our environment.

During this experiment, watching the electrical activity of the brain on EEG would reveal some additional findings. First, the muscles of the body would relax, decreasing the interference of muscle artifact to the signals. What appears like a thick, black smudge overlying the clean signals of the brain waves would go away as sleep begins. In addition, the brain waves themselves would slow. In fact, something called theta activity would be identifiable. Theta refers to the speed of these waves, occurring at a rate of 5 to 8 times per second. This slowing is a clue that the onset of light sleep has occurred.

The Complexity of the Brain

The human brain is one of the most fascinating and least understood elements of our world. Many of its mysteries, including sleep, are just beginning to be unraveled. Much more research is required to clarify even the simplest experiences and functions of this organ. With this being said, some general statements can be made about the nature of consciousness, reflected by both sleep and wakefulness.

There are parts of the brain, including the brainstem and a deep structure called the thalamus, that work to control the entirety of its function. Moreover, these gates affect how the brain controls the body as a whole. These relay stations regulate activity in complex ways. For example, the electrical patterns at the surface of the brain, called the cortex, can be coordinated by the thalamus. It is also here that findings of deeper stage 2 sleep are generated, including slow-wave K-complexes and sleep spindles.

The brainstem has a central role in regulating alertness. Within this structure, the reticular activating system is integral to promoting consciousness. If it is damaged due to trauma or a stroke, a coma may result. It is also in this region that dream-enactment is prevented through active paralysis of the body during REM sleep.

These structures synchronize the activity of the brain, but this may be incomplete. As a result, parts of the brain may have different electrical frequencies. These frequencies correlate to degree of activity, function, and even consciousness. Therefore, sleep may not be an all-or-nothing phenomenon; we are not always all awake or all asleep. It is far more dynamic than this. Rather than being like a light switch – with the brain being on or off – different parts of the brain may be active or quiescent simultaneously. Often associated areas will have their activities coupled, but mixed states can occur.

The Incidence of Sleep Behaviors

With mixed states of consciousness and brain activity, sleep behaviors called parasomnias and other symptoms can occur. It is helpful to consider the overlap between three major states of consciousness and sleep: wakefulness, Non-REM sleep, and REM sleep. These overlaps correspond to distinct sleep disorders.

  • Wakefulness and Non-REM Sleep

If part of your brain is awake, and part of it remains asleep, you may act in ways that can be both bizarre and potentially harmful. If you are unconscious but the part of your brain that controls movement is active, you may get up and not remember it the next day. These conditions typically manifest as sleep behaviors: sleepwalking, sleep talking, sleep eating, and even sleep sex. Episodes may occur out of slow-wave sleep, most often in the first third of the night. Children are commonly afflicted and experience sleep terrors as well.

  • Wakefulness and REM Sleep

When the awake state is mixed with REM sleep, the most unusual events may take place. It may become possible to act out dream content. If you dream about fighting off a robber, you might actually kick or punch an unsuspecting bed partner. This may occur in REM behavior disorder. If consciousness is more fully attained, elements of the dream state may intrude, including symptoms of sleep paralysis: hallucinations, inability to move, fear, and other experiences. The mixed state of sleep paralysis can occur in about 25% of normal people. It may be provoked by other sleep disorders like sleep apnea that can fragment sleep states. Narcolepsy can similarly fragment states of consciousness and may include weakness induced by an emotional response, an occurrence called cataplexy.

  • Non-REM Sleep and REM Sleep

One might surmise that the overlap between Non-REM and REM sleep can occur as well. As the affected person remains unconsciously asleep, and parts of the brain involved in movement may not be activated, these states will go unnoted in the home environment.

It is common to experience mixed states of consciousness. If more elaborate sleep behaviors occur, it may be necessary to identify ways to stabilize the sleep state. Speak with a board-certified sleep specialist about further evaluation and if treatments are necessary.


Kryger, M.H. et al. "Principles and Practice of Sleep Medicine." ExpertConsult, 5th edition, 2011.

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