Fig. 3: LTP/LTD-like plasticity induction in the motor cortex.

a 3D model of the current flow distribution inside the head was calculated based on the finite element method using COMSOL Multiphysics software version 5.2 (details in supplementary information). The electrical current flow induced by 1.0 mA stimulation intensity, and electrode positions C3-Fp2, for anodal (a1, 2, 3), and cathodal (a4, 5, 6) stimulation over the motor cortex is shown. ñ.E refers to the absolute electrical field. b, c post-tDCS cortical excitability alterations after anodal, cathodal, and sham stimulation at the circadian-preferred and non-preferred times in early (b) and late (c) chronotypes (n = 32, 16 per group). The results of the repeated measures ANOVA showed significant interactions of stimulation × chronotype × daytime and stimulation × chronotype × daytime × timeline (see Table 2). The main effects of time of day and chronotype were not significant, however, they significantly interacted. Stimulation and timepoint did not significantly interact with chronotype or time of day (Table 2). Post hoc comparisons (Bonferroni-corrected t tests, two-tailed) of MEP amplitudes to respective baseline values, the sham condition, and respective stimulation conditions at different times of day are marked by symbols in the figures. Filled symbols indicate a significant difference of cortical excitability against the respective baseline values. The floating symbol [*] indicates a significant difference between the real vs sham tDCS conditions, and the floating symbol [**] indicates an additional significant difference between respective timepoints of tDCS conditions at the circadian-preferred vs circadian non-preferred times. Sham stimulation did not induce any significant change in cortical excitability. Data are presented as mean values ± SEM. MEP   motor-evoked potential, C3   left motor cortex, Fp2 right supraorbital area, V/m volts per meter.