Circadian time perception.

The variability of anticipating a meal was investigated. Sprague-Dawley rats earned food by inspecting a food source during a 3-hr interval. Food was not available at other times. In Experiment 1, the meal started 3 or 7 hr after light offset in a 12-hr light-dark cycle. Experiment 2 was conducted in constant darkness with 14-, 22-, 22.5-, 24-, 25.5-, 26-, or 34-hr intermeal intervals. Inspections increased before the meal. Rats timed intervals in the circadian range (22-26 hr) with lower variability than that for intervals outside this range (3-14 and 34 hr). Higher precision in timing selected intervals violates the scalar property. Proximity to a circadian oscillator improves timing precision. Variability may be used to identify oscillators with noncircadian periods.