Rapid phase adjustment of melatonin and core body temperature rhythms following a 6-h advance of the light/dark cycle in the horse.

The research article investigates how quickly horses can adjust their internal biological clocks, particularly the rhythm of melatonin and body temperature, following a sudden shift in their light/dark cycle, similar to experiencing jet lag in humans. It finds that horses rapidly resynchronize their melatonin rhythm, which suggests a robust adjustment mechanism to environmental changes; however, the impact on their athletic performance is yet to be determined.

Objective of the Study

• The research primarily aims to discover how quickly horses’ internal circadian clocks can adjust to a sudden 6-hour change in their light/dark (LD) cycle. This equates to the body clock disruption experienced as jet lag in humans, often resulting in appetite loss, fatigue, disturbed sleep, and performance deficit.
• The impact of such a disruption is particularly crucial for horses, given their role in sports and other physically demanding activities. However, the exact duration and severity of potential jet lag in horses is currently unexplored, which this study aims to uncover.

Methods and Procedure

• The study involved six healthy, 2-year-old mares that were adjusted to a 12-hour light/dark cycle consistent with natural photoperiods.
• The horses lived in a light-proof barn where the lighting schedule mirrored the outside LD cycle. An abrupt advance in the LD cycle was achieved by shortening the subsequent dark period by 6 hours.
• Blood samples were collected and body temperature readings taken at 3-hour intervals for 24 hours on alternate days for 11 days. These helped analyze disturbances to the melatonin and body temperature rhythms.

Results

• The study found horses rapidly adjusted their melatonin rhythm to the 6-hour phase advance of the LD cycle. The adjustment was complete on the first day of the new schedule, and the rhythm remained stable thereafter.
• Contrarily, the horses’ body temperature rhythm took up until the third day to fully adjust. However, the body temperature rhythm pattern was altered for several days following the LD shift.

Conclusion

• The quick resynchronization of the melatonin rhythm suggests horses possess a significantly robust response mechanism to environmental changes.
• Given that only the temperature rhythm was disrupted, the research concludes that the central circadian pacemaker in horses may be particularly strong.
• The study, however, does not comment on the possible consequences such disruptions could have on the horses’ athletic performance, indicating a potential area for further research.