The ubiquitous and rhythmic expression of PER in glia reported here opens more questions regarding the functional significance of glial clocks. The role of glia in locomotor activity rhythms is not clear. Pan-glial knockdown of the circadian clock genes per or cryptochrome via RNAi failed to alter the free-running behavior of adult Drosophila (Ng et al., 2011). Yet, other studies have shown that Drosophila glial cell functions are required for behavioral rhythmicity (Jackson, 2011; Ng et al., 2011, 2016; Jackson et al., 2015). Finally, pan-glia knockdown of several astrocyte enriched genes can cause significant changes in activity level, sensitivity to mechanical stress, and/or alterations in circadian locomotor activity (Ng et al., 2016).