How is stage N1 scored in sleep stage scoring?

Updated: Aug 19, 2019
  • Author: Andres A Gonzalez, MD, MMM, FACNS; Chief Editor: Selim R Benbadis, MD  more...
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Stage N1 is scored when alpha rhythm is attenuated and replaced by low amplitude, mixed frequency signal of 4-7 Hz for more than 50% of the epoch.

Stage N1. Alpha rhythm is attenuated and replaced Stage N1. Alpha rhythm is attenuated and replaced by low-amplitude, mixed-frequency for more than 50% of the epoch.

Stage N1 is also usually characterized by a decrease in muscle tone as detected by chin EMG when compared with Stage W. Other features of Stage N1 include slow eye movements (SEMs; conjugate, sinusoidal eye movements with an initial deflection usually lasting more than 0.5 seconds); vertex sharp waves (which are prominent sharply contoured waves lasting < 0.5 seconds and maximal over the central region), and a low-amplitude mixed frequency EEG signal. Some patients may not generate an Alpha rhythm, even when their eyes are closed while awake. In such patients distinguishing the onset of sleep is more difficult (see below).

Prior to 2007, Stage N1 was the equivalent to R and K stage I, and embodies the transition from the waking to sleeping state. Sleep onset is defined as the first epoch scored as any stage other than Stage W.

For patients who do not generate Alpha rhythms Stage N1 may be scored based on the presence of the following: vertex sharp waves, slow eye movements, or EEG activity in the range of 4-7 Hz with slowing of the background frequencies of the EEG by 1 Hz or more when compared with stage W. SEMs may begin even in wakefulness, thus, stage N1 may be scored earlier in patients without a discernible alpha rhythm.

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