(a) Co-localization of Fos within TH cells. We delivered optical stimuli (25 Hz, during 5 s min-1, for 30 min) to the VTA of eYFP and ChR2 mice at the beginning of the light phase (ZT 2) and sacrificed the mice 90 min later. TH-positive neurons in midline cell groups from hypothalamus to hindbrain were scored for the presence of Fos. **** P < 0.0001, two-way RM ANOVA between distance and viral injection, followed by Bonferroni post hoc tests. The data demonstrate that optical stimulation produced a significant increase in Fos expression in TH+ cells throughout the anterior-posterior length of the VTA, but not in other TH+ neuronal populations. (b) Mean latency (± s.e.m.) to wake from NREM sleep following 5 s stimulation at 25 Hz, 3 days before and following sleep deprivation (SD; 4 h of gentle handling), at the same circadian time, of eYFP (left) and ChR2 (right) mice (n = 6 per group, 1 stim per mouse). P > 0.05, two-tailed paired sample t-tests. (c) EEG power spectrum during NREM sleep from eYFP (left; n = 7) and ChR2 (right; n = 8) mice, 4 s before the onset of stimulation (black) and 4 s into stimulation (gray). Quantification based on the average of 6 stimulations per mouse. (d) EEG power spectrum during wake episodes in eYFP (left) and ChR2 (right) mice during the 6-h stimulation period (ZT 0- ZT 6; light blue), the following 6 h of the light phase (ZT 6 - ZT 12; black), and the subsequent dark phase (ZT 12 – ZT 0; red). Insert represents EEG power spectrum during wake episodes across the 6 h of photostimulation in ChR2 animals. (e) EEG power spectrum of NREM sleep episodes following semi-chronic photostimulation. NREM sleep episodes EEG power spectrum during the 6 h following the termination of photostimulation in eYFP (left) and ChR2 (right) mice. Two-way RM ANOVA between frequency and time, followed by Tukey post-hoc tests. ChR2, P(time) < 0.0001, P(interaction) < 0.0001. eYFP, P(time) > 0.3. The EEG power spectrum during the 1st h following photostimulation differed significantly from the remaining 5 h of the light phase (P < 0.05 in all comparisons; Tukey post-hoc tests). (f) The percentage of time spent awake, in NREM sleep and REM sleep in mice pre-treated (45 min before) with saline / D1,D2 antagonists and undergoing a semi-chronic photostimulation (2 s per min, at 25 Hz, during 2 h; n = 4 per group). ** P < 0.01, **** P < 0.0001, two-way RM ANOVA between compound injected and viral transduction, followed by Sidak’s post hoc tests.