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What’s the Difference Between Sleep and Anesthesia?

  • Published11 May 2021
  • Author Calli McMurray
  • Source BrainFacts/SfN

You’re at the hospital about to get your appendix removed. The anesthesiologist puts an IV in your vein and tells you to count backward from five. “It will feel like you are falling asleep,” they tell you. Before you finish counting, the world fades to black. When you come to, sans appendix, it might feel like only seconds passed, or maybe hours. You’re not sure because you think you were asleep.

But were you actually asleep? Emery N. Brown, a professor of medical engineering and computational neuroscience at the Massachusetts Institute of Technology and professor of anesthesia at Harvard Medical School, is a practicing anesthesiologist at Massachusetts General Hospital. He received the Swartz Prize for Theoretical and Computational Neuroscience in 2020. He explains the difference between being asleep and being under general anesthesia BrainFacts.org

Dr. Emery Brown
Emery N. Brown, professor of medical engineering and computational neuroscience at the Massachusetts Institute of Technology and professor of anesthesia at Harvard Medical School.
MIT Picower Institute

What is anesthesia?

General anesthesia is a drug-induced, reversible coma that has four components:

  1. Analgesia — you don't perceive pain 
  2. Unconsciousness — you're not aware of what's happening 
  3. Amnesia — you’re not forming memories 
  4. Akinesia — you can’t move 

General anesthesia also keeps the body in a stable state by maintaining a consistent blood pressure, heart rate, and body temperature. Regional anesthesia blocks pain perception in a specific area without making the patient unconscious. It’s a common technique for orthopedic surgeries in the lower part of the body, like knee surgery. Sedative drugs block memory formation and make the patient less aware of what’s going on, which is helpful for uncomfortable procedures like colonoscopies.

How does general anesthesia work in the brain?

Anesthetic drugs block the brain’s ability to send information between regions. Many of the commonly used anesthetics bind to GABA receptors, a type of receptor found in inhibitory interneurons all over the nervous system. These interneurons act like routers in a computer network: they connect and modulate all the excitatory neurons in the brain. If you control the interneurons, you can control the rest of the circuits in the brain. Binding to GABA receptors activates interneurons to inhibit the rest of the brain. The enhanced inhibitory activity changes the character of brain waves, the synchronized electrical activity of groups of neurons. They transform from very high frequency, small amplitude waves to very low frequency, large amplitude waves. As long as the patient receives the anesthesia, the brain stays in this state. The brain waves become so structured and regimented that they can't transmit information anymore. As a consequence, brain regions can no longer communicate with each other, resulting in profound unconsciousness and amnesia. The anesthesiologist may administer an opioid and muscle relaxants to ensure adequate analgesia and akinesia, respectively. When the drug wears off, the brain rhythms slowly return to normal, and the patient comes to.

What is the difference between being asleep and being anesthetized?

Sleep and anesthesia are two entirely different conditions. There are two main sleep states: rapid eye movement (REM) sleep and non-REM sleep. The brain and body slow down during non-REM sleep, while REM sleep is very much like an awake state. Through the night, you go between the two states in cycles of about 90 minutes, four to six times total. It’s a natural physiologic process that needs to take place for your mental and physical health.

Anesthesia is nothing like that. During sleep, the brain moves between the slow waves of non-REM sleep and the fast waves of REM sleep. Under general anesthesia, brain waves are held hostage in the same state and remain there for the length of the operation. Then we turn the anesthetics off and allow you to come to.

Do patients ever feel well-rested when they come out of anesthesia?

Coming out of general anesthesia is not the same sensation as waking up from a good night’s sleep. But sometimes, after sedation, people wake up with a good feeling and interpret it as being well-rested. That's because sedative drugs can induce the release of dopamine, which gives you a sense of feeling good.

Can you dream when you are anesthetized?

If you're under full general anesthesia and unconscious, no. But if you're in a lighter state of sedation, then yes, certainly you can dream.

What’s the next step in anesthesia research?

The main thing we're trying to do is better understand how anesthesia works to develop better strategies for producing it, ideally without any side effects like delirium or cognitive impairments. It's a very important area of medicine that gets overlooked because you don't think about it until you need surgery. We need to do more investigations to improve it, because to the extent that we can improve anesthesia care, we can improve surgical care and ICU care. Plus, as we gain a deeper understanding of how the drugs act on the brain, we’ll learn more about how the brain itself works.

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