A pair of photomicrographs showing the distributions of Fos-ir cells in the extended VLPO and VLPO cluster in an animal exposed to light (12% REM sleep, A) and in an animal exposed to dark treatment (30% REM sleep, B) during the early part of the sleep cycle. The counting boxes used for the VLPO cluster and the dorsal and medial extended VLPO are shown in B. These sections are approximately at the level of AP −0.5 in Paxinos and Watson (1986).OC, Optic chiasm.