Humans have internal circadian clocks that ensure important physiological functions occur at specific times of the day. These molecular clocks are regulated at the genomic level and exist in most cells of the body. Multiple circadian resetting cues have been identified, including light, temperature and food. Recently, oxygen has been identified as a resetting cue and emerging science indicates that this occurs through interactions at the cellular level between the circadian transcription-translation feedback loop and the hypoxia inducible pathway (HIF; subject of the 2019 Nobel Prize in Physiology or Medicine). This review will cover recently identified relationships between HIF and proteins of the circadian clock. Interactions between the circadian clock and hypoxia could have wide reaching implications for human diseases and understanding the molecular mechanisms regulating these overlapping pathways may open up new strategies for drug discovery.