Slow eye movements (SEMs) predict EEG power changes and subjective estimates of sleepiness during sleep deprivation only with eyes closed

Objective: The aim of the study is to assess the relationship between slow eye movement (SEM) changes across 37 h of continuous wakefulness and, respectively, EEG power and subjective measures of sleepiness. The relationship has been assessed for each subject and for the entire sample, and comparing eyes-open (EO) and eyes-closed (EC) conditions. Methods: Within a larger study of sleep deprivation, the waking EEG of 19 subjects (10 M and 9 F; age = 24.1 years) was recorded from Cz derivation in 37 sessions at 1 h intervals starting at 10.00 am. Each session consisted of a 2 min eyes-closed period, followed by a 4 min eyes-open period. For the current study, EEG power spectra were calculated in 4-sec epochs across the following bands: delta (1–4 Hz), theta (5–7 Hz), alpha (8–12 Hz), beta1 (13–15 Hz), and beta2 (16– 30 Hz). Horizontal SEMs were scored according to previous studies (1), and subjective measures were provided by the KSS and the VAS for Global Vigor. Results: Day · Hour anovas in the two corresponding 10.00–21.00 intervals pointed to a significantly increased sleepiness consequent to sleep deprivation (i.e., significant main effects for Day) for the subjective measures, and for theta activity and SEMs in both EO and EC conditions. Individual and mean (Z-transformed) cross- correlations of SEMs with EEG and subjective variables across 36 h converged in pointing that oculomotor activity significantly correlates with subjective sleepiness, while EEG power showed an association only with delta and theta power, limited to the EC condition. Conclusions: Similarly to the wake-sleep transition (1), SEMs predict EEG changes only when the relationship is assessed with eyes closed, and taking into account individual differences. Hence, their usefulness in the work place, or in general applied contexts, seems questionable.