Motor cortex excitability upon awakening from REM and NREM sleep: a study by means of transcranial magnetic stimulation.

Introduction: There is increasing evidence that human sleep may be considered as a local process with use-dependent characteristics. For instance, the spectral analysis of sleep EEG showed that NREM sleep is characterized by a fronto-occipital power gradient. Coherently, a larger reduction of frontal regional cerebral blood flow as compared to other cortical areas has been described during sleep. The aim of this study was to assess the motor cortex excitability by applying, for the first time, Transcranial Magnetic Stimulation (TMS) immediately upon awakening from NREM and REM sleep, and recording the corresponding motor evoked potentials (MEPs). Methods: Ten subjects slept for 4 consecutive nights in the laboratory. During the first night (adaptation), motor thresholds (MTs) had been assessed during pre-sleep wakefulness (before lights off), as well as upon awakening from stage 2 and REM sleep. The stimulator was a Magstim 200 Mono Pulse with a figure-of-eight coil with external wing diameter of 9 cm (peak magnetic field: 2.0 T). The MEPs were recorded from the Abductor Digiti Minimi muscles, in both hands. The individual MTs were defined as the lower intensity able to produce at least 3 MEPs with 100 mV of amplitude (peak-to-peak) in 6 consecutive stimulations. Results: A repeated measure ANOVA State (Wake, REM, Stage 2) by Hemisphere (Left, Right) showed a significant main effect for State (F=13.75; p=0.0003), with an increasing linear trend of MTs from wake to Stage 2, with REM sleep showing intermediate values (F=23.63; p=0.001). While during wake and stage 2 MTs were identical, during REM sleep they were higher over the right hemisphere (p=0.04). Conclusions: The increase of MTs upon awakening from stage 2 confirms by an independent method that NREM sleep is characterized by lower levels of brain excitability as compared to REM sleep, whose MTs values are closer to wakefulness levels. During REM sleep, however, the right motor cortex is characterized by a lower excitability than the left one, as indicated by the higher MT over the right hemisphere.