Circadian rhythms are ~24-hour oscillations that provide temporal integration of physiology and behavior that are entrained to the earth’s light-dark cycle via photic input to the suprachiasmatic nuclei (SCN). The SCN (i.e., central clock), sets central and peripheral clocks accordingly to maintain proper synchronization with the external solar day. Exposure to artificial light at night (ALAN) resets the SCN, disrupting daily internal rhythms. Our lab and others have demonstrated daily rhythms in transcellular permeability via efflux mediated transport at the blood-brain barrier (BBB). However, no study has demonstrated daily rhythms in paracellular permeability. We hypothesized that there are daily oscillations in paracellular permeability, and predicted that exposure to ALAN will disrupt these rhythms. Male and female adult Balb/c mice were housed in either a 12:12 light/dark (LD) (150:0 lux) cycle or in ALAN (150:~5 lux) for four weeks. After four weeks, mice received an intraperitoneal injection of NaFl (25mg/kg), a small (376 Da) fluorescent molecule that crosses the BBB primarily via paracellular transport, at one of four timepoints: zeitgeber time (ZT) 0, 6, 12, or 18. Following perfusion, brains were removed and NaFl was extracted. Male mice displayed daily oscillations in paracellular permeability, peaking at ZT6. Whereas, females failed to display a rhythm. ALAN inverted the rhythm in males and significantly increased variability in paracellular transport in female mice. These data detail the importance of sex differences at the BBB and may have implications for chronotherapeutic treatment for CNS diseases.