How Does Zeo Report Stage 4 Sleep?

published by Ryan@Zeo on Tue, 01/17/2012 – 18:10

User Question:
I thought there were four stages of Non REM (NREM) sleep, but you only mention three: Stages 1 and 2 equal Zeo Light Sleep, and Stage 3 equals Zeo Deep Sleep. What happened to Stage 4?

Short Answer :
Non-REM sleep breaks down into two main varieties: Light sleep (stage N1 and N2 sleep) and Deep sleep (N3, known as slow wave sleep). However, since 2007, N4 sleep is no longer recognized as a separate stage by the American Academy of Sleep Medicine (AASM), but rather a “deeper” expression of N3. Since the original Zeo debuted in 2009, its sleep stages were scored according to this new criteria.

There’s many ways to skin a cat – and sleep staging (a.k.a. “sleep scoring”) is no different.  In our quest to understand what’s going on in our brains and how to describe said activity, sleep scientists and researchers had to continually refine their classifications.  The removal of Stage 4 (N4) Sleep is just one step in this fascinating history.

As early as 1937, sleep stages as we’d come to recognize them were created based on brainwave activity (EEG).  The original sleep researchers, Alfred Lee Loomis, E.N. Harvey, and G.A. Hobart seperated these EEG frequencies into five difference groups:  A (alpha); B (low voltage); C (spindle); D (spindle and random); E (random).

They also noticed that we move in and out of stages during the night, but not always in a direct low—>high—>low  or high—>low—->high frequency pattern.

In the 1950s, REM sleep was discovered by William Dement and Nathaniel Kleitman.  They also noted that REM was distinct from other sleep stages –meaning that it had different qualities than other previously observed stages.  In addition, the discovery of REM also introduced the practice of measuring eye movements (EOC) when measuring sleep.

But our modern measure of sleep stages wasn’t codified until 1968, when Allen Rechtschaffen and Anthoney Kales introduced their “R&K sleep scoring manual.”  This is where the five sleep stages – 4 NREM and 1 REM – came into wider use by sleep scientists, researchers and technicians.

Specifically, Rechtshaffen and Kales distinguished between Stage 3 (N3) and Stage 4 (N4) by the following criteria:  Stage 3 included sleep in which less than 50% of activity was composed of Delta waves, while Stage 4 was composed of more than 50% Delta waves.

To recap:

>50% Delta (∂) = Stage 3 (N3) Sleep

<50% Delta (∂) = Stage 4 (N4) Sleep

In 2004, the American Association of Sleep Medicine (AASM) decided to review the R&K scoring system.  It was here that the decision to lump Stage 3 and Stage 4 sleep together under Stage 3 (what Zeo calls “Deep sleep) was made. Researchers concluded that while sleep stages are still relevant for researching the function of sleep, N3 and N4 can (and should) be seen as one distinct stage of slow wave sleep, encompassing the entire range of sleep where Delta (∂) waves occur.

Any % Delta (∂) = Stage 3 (N3) Sleep

These new rules were published in 2007, two years before the debuet of the original Zeo Personal Sleep Coach (a.k.a. Zeo Sleep Manager – Bedside).  As such, we made sure Zeo conformed to these new standards.

While this new scale caused a few changes in normative sleep data, it has not impacted medical diagnostics significantly. So if you go have a professional sleep study done, they will still measure every instance of Delta (∂) sleep, but will report it as Stage 3 (N3) Sleep.

Schultz, Hartmut. Rethinking Sleep Analysis, Journal of Clinical Sleep Medicine. 2008 April 15; 4(2), 99-103
Moser, D., et al., Sleep Classification According to AASM and Rechtschaffen & Kales: Effects on Sleep Scoring Parameters, SLEEP. 2009 February 1; 32(2): 139-149

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