last updated:

April 3, 2021


[Brain Image]    

PSY 340 Brain and Behavior

Class 23: Stages of Sleep & Brain Mechanisms


A. Sleep and Other Interruptions of Consciousness

brain death

Image from Laureys, S. (2005). Death, unconsciousness and the brain. Nature Reviews Neuroscience, 6, 899-909.

B. The Stages of Sleep

[Brain Wave: EEG Tracings]
[Sleep Cycle, revised 2007)

The chart above is a revision based on the AASM Sleep Scoring Manual (Iber et al., 2007; see also Schartzmiller et al., 2010).
It replaces the information in our text.


 Non-REM Sleep includes Stages N1 to N3

  • Stage N1: Mixed frequencies esp. theta waves
  • Stage N2: Sleep spindles and K-complex waves
  • Stage N3: Slow waves (delta) > 20% ("deep sleep")
  • May involve dreaming, but these dreams tend to be less intense than in REM sleep.
  • Constitutes about 75-80% of total sleep time in adults (Porter et al., 2018)

 Stage R (REM ("Rapid Eye Movement") Sleep)
(= "paradoxical" sleep in animals who do not move eyes)

  • Brain waves somewhat similar to awake state or Stage N1 sleep (low-voltage, mixed/fast waves)
  • Rapid eye movements
  • Almost complete loss of muscle control ("atonia")
  • Often, penile erection (males) & vaginal moistening (females)
  • Easy to awaken
  • Usually a time of more intensive, vivid dreams

Sleep Cycle

  • Movement from Stage N1 to Stage N4 and back to Stage N1
  • Stage R (REM Sleep) substitutes for Stage N1 sleep during cycle
  • Between 90-110 minutes to move through an entire cycle
  • Each night = 3 to 5 complete cycles
  • Toward the end of sleeping, individual often does not reach Stage N3

Note that the Sleep Cycle chart above is idealized and somewhat unrealistic. An example of someone who is having trouble sleeping might look like this:

Real life sleep

C. Brain Mechanisms of Wakefulness & Arousal

Sleep & Wakefulness - Brain Mechanisms

Ascending Arousal Systems in the Brain
Structure Function(s) Neurotransmitter(s)
(reticular formation)

  • connected to thalamus & basal forebrain
  • arousal of cortex & increased alertness to new tasks; increases "consciousness"
  • very active during REM sleep
  • damage to the reticular formation leads to decreased arousal
  • Acetylcholine (ACh) & glutamate = excitatory
    basal forebrain
    (anterior to hypothalamus)
    connected to thalamus and cortex
    • excitatory cells using ACh
      • overall arousal, attention to stimuli, increased learning
      • shifts sleep to REM from NREM
    • Inhibitory cells using GABA
      • inhibit thalamus & cortex
      • cells no longer fire spontaneously or in response to slight stimulation

    Acetylcholine (ACh) = excitatory
    [green tracts]

    GABA = inhibitory
    [red tracts]

    hypothalamus (posterior)
    arousal and wakefuilness but mostly inactive during both REM & NREM sleep
    connections throughout the central nervous system to areas involved in wakefulness
    antihistamine drugs may cause drowsiness

    lateral nucleus of hypothalamus releases orexin (= hypocretin) ==> needed to stay awake.
    Histamine (+ Orexin)

    Orexin (Hypocretin) [see below about narcolepsy]
    locus coeruleus
    (dorsal pons)

  • connected to many areas of the cortex
  • suppresses REM sleep when cells in the area stop firing
  • influential in the storage of recent memories

  • Norepinephrine
    dorsal raphe nucleus
    (pronounced ray-fee)

  • Interrupts (turns off) REM sleep
  • inhibits basal forebrain GABA cells (therefore, prevents inhibition in cortex, i.e., it excites)

  • Serotonin

    D. Sleep and the Inhibition of Brain Activity

            Waves in Brain]E. Brain Function in REM Sleep

    F. Sleep Disorders

    These are often called parasomnias ("a category of sleep disorders that involve abnormal movements, behaviors, emotions, perceptions, and dreams that occur while falling asleep, sleeping, between sleep stages, or during arousal from sleep" Wikipedia, 20170317)

    1. Insomnia

    = not sleeping enough due to (1) difficulty falling asleep (onset insomnia), (2) frequent awakening during sleep (maintenance insomnia), or (3) waking up too early (termination insomnia)

    2. Sleep Apnea (in Greek, a = not; pneuma = breath) = intermittent stopping of breathing during sleep due to obstruction of the airway or central nervous system problems

    3. Narcolepsy = frequent, unexpected periods of sleep or sleepiness during the day.

    Includes one or more of these symptoms:

    1. Gradual/sudden onset of sleepiness
    2. Cataplexy: muscle weakness while remaining awake. Often triggered by strong emotions. Occurs occasionally.
    3. Sleep paralysis: inability to move when falling asleep or waking up
    4. Hypnagogic hallucinations: dreamlike experiences at the beginning of sleep.

    Equivalent to REM-like sleep intruding into the awake state

    Treated with stimulants, e.g., Ritalin or Cylert

    4. Periodic Limb Movement Disorder

     = during NREM sleep individual moves legs and sometimes arms every 20-30 seconds for minutes or hours.
    5. REM Behavior Disorder

    = during REM sleep individual moves vigorously or violently (kick, punch, etc.); dreams of violent nature
    6. Night Terrors

     = an experience intense anxiety involving crying and screaming, but often not actually awakening.
    7. Sleep Talking
    8. Sleepwalking


    Ad Hoc Committee of the Harvard Medical School (1968). A definition of irreversible coma: Report of the ad hoc committee of the Harvard Medical School to examine the definition of brain death. Journal of the American Medical Association 205, 337340.

    Baglioni et al. (2011) Insomnia as a predictor of depression - A meta-analytic evaluation of longitudinal epidemiological studies Journal of Affective Disorders, 135, 10-19.

    Hertenstein et al. (2019) Insomnia as a predictor of mental disorders - A systematic review and meta-analysis Sleep Medicine Reviews, 43, 96-105.

    Iber, C., Ancoli-Israel, S., Chesson, A., & Quan, S. F. (2007). The AASM manual for the scoring of sleep and associated events. Westchester, IL: American Academy of Sleep Medicine.

    Makin, S. (2019, March 12). A genetics basis for insomnia emerges from the twilight. Scientific American [online]. Retrieved from https://www.scientificamerican.com/article/a-genetic-basis-for-insomnia-emerges-from-the-twilight/

    Porter, R. S.,Kaplan, J. L, Lynn, R. B., & Reddy, M. T. (Eds.). (2018).  The Merck manual of diagnosis and therapy (20th ed). Kenilworth, NJ: Merck Sharpe & Dohme Corp.

    Rechtschaffen, A. & Kales, A. (1968). A manual of standardized terminology, techniques, and scoring system for sleep stages of human subjects. National Institutes of Health Publication No. 204. Washington, DC: U.S. Government Printing Office.

    Rechtschaffen, A., & Siegel, J. M. (2000). Sleep and dreaming. In E. R. Kandell, J. H. Schwartz, & T. M. Jessel (Eds.), Principles of neuroscience (4th ed.; pp. 936-947). New York: McGraw-Hill. Retrieved 3/5/05 from Jerome Siegel's Web site: http://www.npi.ucla.edu/sleepresearch/SleepDream/sleep_dreams.htm

    Shatzmiller, R. A., Gonzalez A. A., Ko, D. Y., & Zeidler, M. R. (2010). Sleep stage scoring. WebMD: eMedicine Neurology. http://emedicine.medscape.com/article/1188142-overview

    Siegel, J. M. (2004). Hypocretin (Orexin): Role in normal behavior and neuropathology. Annual Review of Psychology, 55, 125-148. doi:10.1146/annurev.psych.55.090902.141545

    World Health Organization (WHO). (2009). Pharmacological Treatment of Mental Disorders in Primary Health Care. Geneva, Switzerland.

    This page was first posted March 6, 2005.