Since the free-running τ is close to, but not exactly, 24 h, the internal clock will dissociate from the natural 24 h day unless a specific mechanism keeps it synchronized on a day-to-day basis. In these cases τ was 22.88 h in the BALB/c mouse and 23.88 h in the C57BL/6 mouse. The final 5 days of entrainment to an LD 12:12 cycle are shown (scotophase shaded) before the mice were placed in constant darkness. Double-plotted actograms of wheel-running activity of two inbred mouse strains to show the variability of the free-running period (τ). Each cycle of the free-running circadian rhythm can be divided into 24 h of circadian time (CT), with CT0 being defined as onset of the subjective day and CT12 as onset of the subjective night.įigure 2.12.5. Under free-running conditions the activity period deviates from exactly 24 h and consequently drifts to the left if τ is less than 24 h, and to the right if it is greater than 24 h. This allows for good visual comparison of individual activity patterns of mice and also reveals characteristic differences between strains (see Figure 2.12.2 and also Figure 2.12.5).
More complex and longer recordings are often presented as double plots where two consecutive days (48 h) are plotted side-by-side in one line and the second day is repeated in the next line underneath the first day, followed by the data of the third day next to it, and so on. Such activity recordings are usually depicted by plotting the data of consecutive days underneath each other. If such a recording is done under a regular LD cycle (most often in mice LD 12:12) the rhythm is entrained and shows a stable phase relationship (Ψ) with the zeitgeber ( Figure 2.12.1). Running wheels can easily monitor locomotor activity of mice passive infrared detectors, photodiodes or implantable transponders, and thus long-term recordings that do not disturb the animals, are possible. This behaviour has most likely evolved as an avoidance response to predation pressure by birds of prey which are primarily active during daytime. A nocturnal animal species such as the house mouse is primarily active during the night-time and most of its exploratory activity, feeding and drinking is done during the hours of darkness.
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