Circadian clocks coordinate behavioral and physiological processes with daily 
light-dark cycles by driving rhythmic transcription of thousands of genes. 
Whereas the master clock in the brain is set by light, pacemakers in peripheral 
organs, such as the liver, are reset by food availability, although the setting, 
or "entrainment," mechanisms remain mysterious. Studying mouse fibroblasts, we 
demonstrated that the nutrient-responsive adenosine monophosphate-activated 
protein kinase (AMPK) phosphorylates and destabilizes the clock component 
cryptochrome 1 (CRY1). In mouse livers, AMPK activity and nuclear localization 
were rhythmic and inversely correlated with CRY1 nuclear protein abundance. 
Stimulation of AMPK destabilized cryptochromes and altered circadian rhythms, 
and mice in which the AMPK pathway was genetically disrupted showed alterations 
in peripheral clocks. Thus, phosphorylation by AMPK enables cryptochrome to 
transduce nutrient signals to circadian clocks in mammalian peripheral organs.