The Cryptochrome 1 ( Cry1 )–deficient duper mutant hamster has a short free-running period in constant darkness (τ DD ) and shows large phase shifts in response to brief light pulses. We tested whether this measure of the lability of the circadian phase is a general characteristic of Cry1 -null animals and whether it indicates resistance to jet lag. Upon advance of the light:dark (LD) cycle, both duper hamsters and Cry1 −/− mice re-entrained locomotor rhythms three times as fast as wild types. However, accelerated re-entrainment was dissociated from the amplified phase-response curve (PRC): unlike duper hamsters, Cry1 −/− mice show no amplification of the phase response to 15’ light pulses. Neither the amplified acute shifts nor the increased rate of re-entrainment in duper mutants is due to acceleration of the circadian clock: when mutants drank heavy water to lengthen the period, these aspects of the phenotype persisted. In light of the health consequences of circadian misalignment, we examined effects of duper and phase shifts on a hamster model of heart disease previously shown to be aggravated by repeated phase shifts. The mutation shortened the lifespan of cardiomyopathic hamsters relative to wild types, but this effect was eliminated when mutants experienced 8-h phase shifts every second week, to which they rapidly re-entrained. Our results reveal previously unsuspected roles of Cry1 in phase shifting and longevity in the face of heart disease. The duper mutant offers new opportunities to understand the basis of circadian disruption and jet lag.