One of the important findings of our study is that, with the exception of the central brain neuropil glia, all PER expressing glial subtypes display significant age-related decline in PER protein levels. These data are consistent with the age-related decrease in PER protein reported by Western blot in whole heads (Luo et al., 2012; Rakshit et al., 2012; Kuintzle et al., 2017). Previous immunofluorescent studies detected PER decline in the retinal photoreceptors of old flies (Luo et al., 2012; Rakshit et al., 2012). Our data now show that similar decline occurs in the majority of glial cells. Although the reasons for age-related decrease in PER protein shown here are not known, it could be related to reported reduction of TIM protein in the heads of old flies (Luo et al., 2012; Rakshit et al., 2012). TIM protein is known to be required for PER stability (Hardin, 2011). It is also well known that PER is an essential repressor of CLK/CYC-activated circadian transcription of target genes (Hardin, 2011); therefore, our data suggest that the repressive arm of the circadian clock weakens in glia during aging due to decline of nuclear localized PER protein. Consistent with this hypothesis, a recent RNA-seq study showed that per mRNA expression is higher in the heads of old flies compared to young while protein levels are decreased in old (Kuintzle et al., 2017).