Circadian rhythm
= Circadian rhythm
{wiki}
A <circadian rhythm> is an endogenous biological <oscillation> with a <period> near 24 hours, persisting without a repeating external signal. A <circadian clock> times biological activities and anticipates daily changes without continual external instructions. A <free-running circadian rhythm> reveals its intrinsic <period>; <entrainment of circadian rhythm> coordinates it with an external cycle.<Overview of Circadian Rhythms by Martha Hotz Vitaterna and others>[Vitaterna, Takahashi and Turek (2001), Overview of Circadian Rhythms, pp. 85–86, Biological Rhythms and Basic Properties of Circadian Rhythms]{ref}
In 1994, <Martha Hotz Vitaterna> and colleagues screened offspring of mutagen-treated C57BL/6J mice (the <house mouse>). Each mouse lived in a cage with a running wheel, inside a ventilated, light-tight chamber with timer-controlled lighting. Wheel revolutions were recorded electronically or on an event recorder. The initial screen used 14:10 LD for the first 41 animals and 12:12 LD thereafter; LD denotes hours of light and darkness. Transfer to constant darkness, DD, omitted the next scheduled lights-on; recording continued for at least three weeks. The first 41 animals also received brief light pulses, so those records were not uninterrupted DD.<Mutagenesis and Mapping of a Mouse Gene, Clock, Essential for Circadian Behavior by Martha Hotz Vitaterna and others>[Vitaterna and others (1994), Mutagenesis and Mapping of a Mouse Gene, Clock, Essential for Circadian Behavior, Figure 1 and References and Notes 6–10]{ref}
The subsequent-generation comparison in Figure 2 used 12:12 LD before DD. Animals without the <Clock mutation> and with mutant <genotypes> supplied the comparison: the B6 examples in panels A–C had initial DD <periods> of 23.6, 24.8 and 27.1 hours, respectively; the <homozygote> later lost its sustained <circadian rhythm>. Total activity remained comparable across <genotypes> in the interstrain comparison. The <mutation> therefore altered timing, beyond merely reducing running. Wheel activity is a behavioral readout; it does not directly measure every <circadian clock> or establish a human <period>.<Mutagenesis and Mapping of a Mouse Gene, Clock, Essential for Circadian Behavior by Martha Hotz Vitaterna and others>[Figure 2A–C; Table 2 and discussion of activity counts and loss of rhythmicity; References and Notes 10 and 14]{ref}
Daily activity under LD alone cannot establish <entrainment of circadian rhythm>. Light can directly change activity without resetting the <circadian clock>: <masking of circadian rhythm>. Removing the light cycle helps distinguish these effects; they can coexist.<Overview of Circadian Rhythms by Martha Hotz Vitaterna and others>[p. 92, glossary entries Entrainment, Free running and Masking]{ref} In an experiment, other repeating inputs must also be controlled: a nominally dark room with daily handling is not automatically an environment without timing cues.