Stage 3 deep non-REM sleep has less than 50% deep slow brainwave activity, while stage 4 has more than 50%.

“from there you start to go down to the really deep stages of non REM sleep stages three and four and at that point the brain goes into this incredible synchronous mantra chant of these slow waves so the brain waves slow down you go down to maybe just one or two cycles per second very very slow brainwave activity but the size of those waves crashing on the beach of the cortex as it were they are huge and the reason is this go back to the sports stadium analogy let's say that we're here at Berkeley and were playing Stanford arch-rivals which hopefully you're gonna kick their butt fingers crossed make sure there's no band anywhere to be found it is a terrible to serve but at that point they're chanting in unison chanting so it's louder but they're now making the same sound together and you can actually hear what's being said exactly so now in that single microphone you can hear Stanford sucks mm who's Stanford sucks so all of a sudden all of for reasons that we are only now starting to understand the brain coordinates hundreds of thousands of neurons unlike it does at any moment elsewhere in the 24-hour period hundreds of thousands of brain cells all decide to join hands as it were physiologically metaphorically and they all fire together and then they all go silent they all fire together and then they all go silent”

“the way it's defined is that what proportion of a 30 second period of time and that's the way that we score sleep I'll have you sleep for 8 hours then I'll chop up that 8 hours into thirty-second bits of information and for each 30 seconds you'll get a sleep score and then we add them all up and what differentiates stage 3 of deep non REM sleep from the very deepest sage which is stage 4 non REM sleep is simply what proportion of that 30 seconds is consumed by that deep slow brainwave activity if it's sort of less than 50% then it's stage 3 if it's more than 50% stage 4”

“morphologically on average yes the waves are typically larger in amplitude and slower in frequency in stage four than they are in stage three”

“what differentiates stage three of deep non-rem sleep from the very deepest stage which is stage four non-rem sleep is simply what proportion of that 30 seconds is consumed by that deep slow brainwave activity if it's sort of less than 50 percent then it's stage 3 if it's more than fifty percent stage four”