And the circadian rest-activity cycle

Some of the most remarkable advances in elucidating the molecular basis of circadian rhythms have been made in mutants of the fly Drosophila. Two types of circadian rhythm are known in that organism. The first affects the rest-activity cycle, and the second underlies the daily eclosion peaks of pupae. Both types of rhythm persist in constant light or temperature conditions (Pittendrigh, 1960,1961). 

If the model for the oscillator is correct. Per and Cry expression will define solar/ circadian time, driving the activity—rest cycle rather than just being a passive reflection of the activity cycle. Therefore, their expression patterns should exhibit the same phase in the SCN of nocturnal and diurnal species. This is confirmed by examination of Per expression in the SCN of the diurnally active ground squirrel, Spermophilus (Mrosovsky et al 2001). The rhythm of Per1 and Per2 expression in... 

The sinusoidal output signal produced by the SCN can be described by its period (cycle length), phase (position in the cycle), and amplitude (range between highest and lowest signal). The output amplitude reflects the strength or robustness of the circadian timing system, which can also be described as the drive to restore homeostasis in response to stimuli or the extent to which circadian behavior is separated into distinct periods of activity and rest within one cycle [1-3]. 

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