BACKGROUND: Up to now, sleep stages have traditionally been determined by the visual inspection of individual EEG waves. However, the exact physiological meaning of the sleep waves is not known. The purpose of this study was to try and find out the physiological parameters of the EEG of the sleep and wakefulness states by calculating one of the non-linear chaos parameter, the largest Lyapunov exponent (LLE), of EEG time series. METHODS: The digital EEG of the wakefulness with eye opening (WEO), wakefulness with eye closure (WEC), stage1 (S1), stage2 (S2), stage3 or 4 (S34) were recorded at centroparietal region (C4-P4 bipolar derivation) in 10 normal subjects. Lyapunov exponents of 50 EEG time series in different states were compared. RESULTS: LLE's of WEO, WEC, S1, S2, S34 showed an increas-ing tendency as states switched from wakefulness to sleep. LLE of sleep was larger than that of awake state. CONCLUSIONS: The EEG of the sleep state appeared to be more chaotic than that of the awake state. This nonlinear chaos parameter can be used as a physiological parameter of normal sleep and awake states.
Electroencephalography* ; Sleep Stages ; Wakefulness*