Does more sleep matter? Differential effects of NREM- and REM-dominant sleep on sleepiness and vigilance

We investigated effects of NREM and REM predominant sleep
periods on sleepiness and psychomotor performances measured
with visual analog scales and the psychomotor vigilance task,
respectively. After one week of stable sleep-wake rhythms, 18
healthy sleepers slept 3 h of early sleep and 3 h of late sleep, under
polysomnographic control, spaced by two hours of sustained
wakefulness between sleep periods in a within subjects split-night,
sleep interruption protocol. Power spectra analysis was applied for
sleep EEG recordings and sleep phase relative power proportions
were computed for six different frequency bands (delta, theta, alpha,
beta, gamma). Both sleep periods presented with similar sleep
duration and efficiency. As expected, phasic NREM and REM
predominances were obtained for early and late sleep conditions
respectively. Albeit revealing additive effects of total sleep duration,
our results showed a systematic discrepancy between psychomotor
performances and sleepiness levels. In addition, sleepiness
remained stable throughout sustained wakefulness during both
conditions, whereas psychomotor performances even decreased
after the second sleep period. Disregarding exchanges for frequency
bands in NREM or stability in REM, correlations between outcome
measures and EEG power proportions further evidenced directional
divergence with respect to sleepiness and psychomotor performances
respectively. Showing that the functional correlation pattern
changed with respect to early and late sleep condition, the relationships
between EEG power and subjective or behavioral outcomes
might however essentially be related to total sleep duration