CLOCK and Arrhythmia: While both genetic lines demonstrate some level of disrupted circadian rhythmicity in activity and clock machinery, Clock mutants have severe circadian arrhythmia under constant darkness [44] and reduced expression of circadian mRNA transcripts [45,46,47] in the suprachiasmatic nucleus, whereas Rev-erbα knockouts show a more modest effect on periodicity [48] and elevated circadian clock machinery transcript levels [35,49,50], including Bmal1.