Daily rhythms exist across phyla and in the regulation of many molecules, cells, tissues, and behaviors.  This talk will review evidence that brain cancers (glioblastoma) have circadian rhythms in gene expression and sensitivity to chemotherapy that synchronize to the body.  This creates an opportunity for improved therapy. Using treatments that target glucocorticoid signaling to the tumor, we find that we can dramatically improve glioblastoma outcomes in mice.