Effects of sleep loss on delta (0.3-3 Hz) EEG and eye movement density: new observations and hypotheses.

One night's sleep loss in young adults increased delta (0.3-3 Hz) EEG only in the first non-REM period of recovery sleep. The delta increase was limited to frequencies 0.3-4 Hz; within this range, the effects on wave form periods and amplitudes differed by frequency band. These results illustrate the value of computer analysis applied to the physiological units of sleep (the successive non-REM and REM periods of each sleep cycle). The finding that all of the delta increase occurred in the first sleep cycle appears inconsistent with the exponential decline of delta across cycles predicted by 'recovery' models of sleep. The fact that wave periods and amplitudes are differentially affected by sleep loss indicates that it is premature to adopt any single wave form characteristic (e.g., power spectral density) to index delta sleep. Our data also confirm a recent report that eye movement density decreases after sleep loss; we hypothesize that this change results from greater depth of sleep; an inverse relation of depth of sleep to eye movement density provides a coherent explanation for a range of otherwise disparate observations. Lastly, we propose a new hypothesis to account for the presence of eye movement during REM sleep.