Many molecular, cellular, and physiological processes in most organisms are coordinated with the predictable changes of the 24-h solar day. The circadian clock provides the mechanism of time keeping that is based on a negative feedback loop of transcriptional activators and repressors that generate endogenous molecular oscillations of circa 24 h (Hardin, 2011). A core repressor in the clock mechanism is encoded by the gene period (per) and the translocation of the PER protein into the cell nuclei followed by its degradation is the fundamental feature of clock function (Hardin, 2011). In both mammals and Drosophila, the circadian system consists of central and peripheral clocks (Hardin and Panda, 2013). Central pacemaker neurons in mammals are located in the suprachiasmatic nuclei (SCN). The central pacemaker neurons driving behavioral rest/activity rhythms consist of a network of about 150 neurons in the Drosophila brain. In addition to these central pacemaker neurons, mammals have intrinsic peripheral clocks in cells of fat tissues, kidneys, liver, and most other organs. Many of these peripheral tissues that express autonomous oscillators coordinate local tissue-specific processes. Similarly to mammals, peripheral clocks are widespread in fly tissues and function independently of the central pacemaker coordinating tissue-specific physiological processes (Giebultowicz, 2001). Within the nervous system, peripheral clocks are present in retinal photoreceptor cells and in other sensory neurons. In addition to neurons, some glial cells rhythmically express circadian clock genes in both mammals (Prolo et al., 2005; Marpegan et al., 2011; Hayashi et al., 2013) and in Drosophila (Ng et al., 2011). Early studies showed that PER protein is expressed in non-neuronal cells (Zerr et al., 1990; Ewer et al., 1992) and suggested that PER expression in these presumed glia is sufficient for manifestation of behavioral rhythmicity (Ewer et al., 1992). However, which glial cell subtypes express the PER-based oscillator and what their roles may be in the timekeeping processes remain poorly understood.