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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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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 –  ICanFeelIt Jun 17 '13 at 8:09
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. –  Ana Jun 17 '13 at 12:36
@Ana I will have to look up the criteria again. –  Chuck Sherrington Jun 17 '13 at 15:25
@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) –  Chuck Sherrington Jun 17 '13 at 20:08
Thanks! I guess this is quite different from doing a time-frequency analysis, and then concluding about delta power. –  Ana Jun 17 '13 at 20:13
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