It has been established that some glial subtypes express circadian clock genes in a rhythmic manner. In mammals, both astrocytes (Prolo et al., 2005; Marpegan et al., 2011) and microglia (Hayashi et al., 2013; Fonken et al., 2015) rhythmically express Per1 and Per2 proteins. Cultured astrocytes from Per1::luciferase transgenic rats and knock-in mice are capable of maintaining modest rhythms in circadian clock gene expression that can be entrained by physiologically relevant temperature changes (Prolo et al., 2005). Rhythmic expression of several clock genes was also shown in cortical microglia by qRT-PCR (Hayashi et al., 2013; Fonken et al., 2015). Expression of the circadian clock gene per in glia have been also suggested in flies (Ewer et al., 1992) and this was confirmed more recently although, it is not clear which glial subtypes express PER protein (Ng et al., 2011).