In humans, delta wave sleep ("slow wave" sleep, stages III and IV by AASM criteria) is more common in children and becomes only a small percentage of total sleep time in adults.

Cortical EEG waves have notably low frequencies (sometimes as low as 1Hz) during these stages. I've always modeled this in my own understanding as the brain "polling" the environment at a lower rate, but why do stimuli that coincide directly with the peaks of the cortical waves still not seem to arouse or awaken the person sleeping? Clearly there are some physiological "shunts" occurring in the brainstem and cortex to dampen the effects of external stimuli, but why see any activity at all, then?

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    $\begingroup$ I can remember from physiology of sleep lesson that there is some chemically division in deep sleep between cortex and subcortex. I can't find reference. Also it could be speculated that hippocampal theta is often seen in children because of consolidation of memory. ncbi.nlm.nih.gov/pubmed/8124075 $\endgroup$
    – ICanFeelIt
    Jun 17, 2013 at 8:09
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    $\begingroup$ Delta waves are always present to some degree, usually with more power than higher frequencies. I wonder, do you know whether delta wave sleep refers to delta waves increasing in amplitude, or to other waves not being as prominent as during other sleep phases? The first would (most likely) imply integration of slow brain activity over a larger number of brain areas, whereas the second would imply silencing of various types of cognitive processing. $\endgroup$
    – Ana
    Jun 17, 2013 at 12:36
  • $\begingroup$ @Ana I will have to look up the criteria again. $\endgroup$ Jun 17, 2013 at 15:25
  • $\begingroup$ @Ana Stage III is defined as <50% delta activity (so activity like spindles, etc. are allowed), and stage IV requires >50% delta activity (see neurores.wikidot.com/stage-iii-iv-sleep-eeg) $\endgroup$ Jun 17, 2013 at 20:08
  • $\begingroup$ Thanks! I guess this is quite different from doing a time-frequency analysis, and then concluding about delta power. $\endgroup$
    – Ana
    Jun 17, 2013 at 20:13

1 Answer 1


Slow EEG waves reflect a slow oscillation in cortical neurons, between a depolarized state (UP-State) and hyper polarized state (DOWN-state; Steriade et al., 1993a, 1993b). During the down-state, neurons are disfacilitated (they can't really be activated by external stimuli or depolarized; Contreras et al., 1996). As the brain progress into deep sleep (lots of large-amplitude slow waves), not only cortical but also thalamic neurons becomes more and more hyper polarized (Amzica & Steriade, 2001). Steriade (2006) proposed that this progressive hyper polarization that is occurring during deep sleep might protect the brain from interference from the external world and facilitate memory consolidation, because the brain becomes "non-responsive" to the external environment. Indeed, this thalamo-cortical hyper polarization occuring during the down-state might "gate" the brain from external influence.

Recent neuroimaging studies in humans (Schabus et al., 2012) indeed showed that during the DOWN state, there was no cortical response to auditory stimuli in fMRI. Thus we think that those waves, either in N2 (K-Complex) or N3 (SW), protect the sleep brain from external influence.

Sources :

  1. Steriade M, Nuñez A, Amzica F. A novel slow (< 1 Hz) oscillation of neocortical neurons in vivo: depolarizing and hyperpolarizing components. Journal of Neuroscience. 1993;13(8):3252–3265.

  2. Steriade M, Contreras D, Curró Dossi R, Nuñez A. The slow (< 1 Hz) oscillation in reticular thalamic and thalamocortical neurons: scenario of sleep rhythm generation in interacting thalamic and neocortical networks. J Neurosci. 1993;13(8):3284–3299.

  3. Contreras D, Timofeev I, Steriade M. Mechanisms of long-lasting hyperpolarizations underlying slow sleep oscillations in cat corticothalamic networks. J Physiol (Lond). 1996;494 ( Pt 1):251–264.

  4. Amzica F, Steriade M. Electrophysiological correlates of sleep delta waves. Electroencephalography and Clinical Neurophysiology. 1998;107(2):69–83. doi:10.1016/S0013-4694(98)00051-0.

  5. Steriade M. Grouping of brain rhythms in corticothalamic systems. Neuroscience. 2006;137(4):1087–1106. doi:10.1016/j.neuroscience.2005.10.029.

  6. Schabus M, Dang Vu TT, Heib DPJ, et al. The Fate of Incoming Stimuli during NREM Sleep is Determined by Spindles and the Phase of the Slow Oscillation. Front Neurol. 2012;3:40. doi:10.3389/fneur.2012.00040.

  • $\begingroup$ Please cite all of the sources that you have used. Otherwise, this is a great answer. $\endgroup$ Mar 6, 2015 at 6:05
  • $\begingroup$ You didn't need to make it community wiki, it was fine. $\endgroup$ Mar 7, 2015 at 1:03
  • $\begingroup$ This was actually a mistake, I hitted the button. But as i've just learned, I guess it's too late now. $\endgroup$ Mar 7, 2015 at 2:07

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