Detection of electroencephalogram features of sleep stage 2 by a new scoring system

The aim of this study was to clarify the electroencephalogram (EEG) characteristics of sleep stage 2 (S2) by using a new scoring system of 5s-EEG stages. Twelve healthy university students (nine females and three males, 20 to 24 years) with no nocturnal sleep complaints took a nap for 1 h from 14:00. EEG during S2 from the onset of the first spindle to sleep stage 3 was manually scored into 5-s epochs of flattening, theta, spindle, K-complex, fast delta, and slow delta stages, according to criteria for EEG stages used for hypnagogic period and REM sleep. EEG stages of flattening, theta, and fast delta occupied 97% of S2. Cumulative epochs of EEG stages of theta and fast delta significantly increased as time elapsed from S2 onset. These results suggest that EEG components of flattening, theta, and fast delta constitute the background of S2, and that the quality of S2 may be accounted for by cumulative epochs of theta or fast delta. Further studies are required to examine whether the EEG features observed during a nap could be applicable to nocturnal sleep.