The mammalian master clock driving circadian rhythmicity in physiology, 
metabolism, and behaviour resides within the suprachiasmatic nuclei (SCN) of the 
anterior hypothalamus and is composed of intertwined negative and positive 
autoregulatory transcription-translation feedback loops. The Cryptochrome 1 and 
2 gene products act in the negative feedback loop and are indispensable for 
molecular core oscillator function, as evident from the arrhythmic wheel running 
behaviour and absence of cyclic clock gene expression in mCry1/mCry2 double 
mutant mice in constant darkness. Recently, we have measured real-time 
multi-unit electrode activity recordings in hypothalamic slices from 
mCry-deficient mice kept in constant darkness and observed a complete lack of 
circadian oscillations in firing patterns. This proves that CRY proteins, and 
thus an intact circadian clock, are prerequisite for circadian rhythmicity in 
membrane excitability in SCN neurons. Strikingly, when mCry-deficient mice are 
housed in normal light-dark cycles, a single non-circadian peak in neuronal 
activity can be detected in SCN slices prepared two hours after the beginning of 
the day. This light-induced increase in electric activity of the SCN suggests 
that deletion of the mCry genes converts the core oscillator in an 
hour-glass-like timekeeper and may explain why in normal day-night cycles 
mCry-deficient mice show apparently normal behaviour.