The boundary between wakefulness and sleep: quantitative electroencephalographic changes during the sleep onset period.
Level 4 - case-series / case-control
Single-cohort observational neurophysiological study without a comparator group.
PubMed 11744241 · doi:10.1016/s0306-4522(01)00309-8
What was done
Sleep recordings from 26 normal subjects were evaluated using quantitative electroencephalography (qEEG) across 1–28 Hz at 1-Hz bin resolution. Individual time series were aligned to two distinct markers of sleep onset—the first occurrence of stage 1 and the first occurrence of stage 2—to compare spectral power changes and trajectories across the wakefulness-to-sleep transition.
What was found
During the stage 1 transition, power increased linearly in the 1–6-Hz range and decreased in the 9–12-Hz and 16–28-Hz ranges. During the stage 2 transition, power increased linearly in the 1–7-Hz and 14–15-Hz ranges, decreased in the 18–28-Hz range, and fit a second-order polynomial curve in the 8–12-Hz range. Principal component analysis grouped frequency bands into delta/theta/sigma (1–7 and 12–16 Hz), beta (17–28 Hz), and alpha (8–11 Hz); alpha power showed a second-order polynomial trajectory with a minimum during stage 1 and subsequent increase in stage 2. Specific numerical power values, standard deviations, and exact p-values were not reported in the abstract.
Why it matters
This study provides microstructural spectral evidence that stage 2 sleep onset represents a more coherent and functionally distinct physiological demarcation of sleep onset than stage 1.
Limits
The study is limited by a small sample size (n = 26) of healthy subjects, limiting generalizability to populations with sleep disorders or altered sleep architecture. The abstract omits exact numerical power measurements, variance data, and demographic characteristics.