Meal Timing Dominates the Lighting Regimen as a Synchronizer of the Circadian Blood Ethanol Clearance Rate Rhythm in Rats

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Abstract
The effects of timing of a single daily meal on the circadian rhythm of blood ethanol clearance rates were investigated in two groups of rats maintained on opposite 12 hours light:12 hours dark (LD) schedules. Initially, the rats were fed ad libitum. Then feeding was restricted to 4 mid-light (ML) hours for one group and 4 mid-dark (MD) hours for the other. Finally, the meal timing was reversed; the ML-fed group became the MD-fed group and vice versa. Following acclimatization to each of the 3 feeding regimens, blood ethanol clearance rates were determined for subgroups (n = 8) of both groups injected (i.p.) with ethanol at 5 times during 24-hour spans. The LD schedule synchronized the clearance rate rhythm during ad libitum feeding. Although the rhythm persisted without significant amplitude changes during restricted feeding regimens, minimal clearance rates during ML and MD feeding approximated the time of food presentation, with maximal rates 8–12 hours later. This relationship remained constant when the feeding phases were reversed. Thus, when food availability is limited, the single daily meal dominates the lighting regimen as synchronizer of the circadian blood ethanol clearance rate rhythm.