Experimental jetlag disrupts circadian clock genes but improves performance in racehorses after light-dependent rapid resetting of neuroendocrine systems and the rest-activity cycle.

The research article explores how a simulated jetlag phase shift in light exposure impacts the performance of racehorses. Interestingly, while it temporarily disrupts the animals’ biological clocks, it significantly improves their athletic performance without causing stress.

Phase Shift and Circadian Rhythms

• The researchers created a jetlag-like condition for the racehorses by suddenly changing their 24-hour light/dark cycles. In humans, such changes (often caused by transmeridian air travel) could disrupt circadian biological rhythms. This usually results in detrimental effects on cognitive and physical performance.
• Contrarily, in horses, this phase shift resulted in a temporary impairment of the circadian peaks of peripheral clock gene expression but led to an acute enhancement in athletic performance.

Improved Performance Quantified

• The study found that both aerobic and anaerobic capacities got significantly increased due to this phase-scheduling. This enhancement allowed the horses to stay active for a longer duration before fatigue set in.
• Notably, these significant physical enhancements did not lead to any stress in the horses, as would typically be the case in humans.

Hormonal Responses and Early Adaptation

• Alongside the improved physical performance, there was quick restoration or ‘re-entrainment’ of the molecular clockwork and the circadian pattern of melatonin. There was no disruption of the adrenal cortical axis, but a well-timed increase of prolactin was observed. Prolactin is a hormone known to influence organs crucial for physical performance.
• In the absence of a structured light/dark cycle, the daily rhythm of locomotor activity was eliminated completely. However, it was promptly restored upon reintroduction of the light/dark cycle, pointing to a fast mechanism of adaptation and reliance on light exposure for daily diurnal activity.

Role of Light and Concluding Remarks

• The results from the study indicate that horses are particularly sensitive to sudden changes in light exposure. Unlike in humans, they appear to benefit from these changes.
• The benefit likely arises from the powerful influence of light, which facilitates a fast and advantageous adjustment process in response to the phase shift.