The consequences of jet lag for the equine athlete have become more relevant in recent times due to increased travel of performance horses across multiple time zones for international competition. The effects of jet lag are significantly more detrimental for the professional athlete hoping to perform optimally in a new time zone. Before defining the implications of jet lag for the horse, it is first necessary to understand the effects of light on any mammalian system. Most all life on earth is influenced by the daily cycles of light and dark brought about by the presence of the sun and the continuous rotation of our planet around its own axis.
From the simplest algae to mammals, nearly all organisms have adapted their lifestyle in such a way that they organize their activities into 24-hour cycles determined by sunrise and sunset. For this reason, many aspects of physiology and behaviour are temporally organized into circadian rhythms driven by a biological clock. Thus, biological clocks have evolved that are sensitive to light and so enable physiological anticipation of periods of activity. Light is the primary cue serving to synchronize biological rhythms and allows organisms to optimise survival and adapt to their environment. An example of this environmental adaptation is clearly evident in the mare’s natural breeding season. As the number of hours of light gradually increases in the early spring, the mare’s reproductive system reawakens and within weeks is ready for conception.
With an 11 month, one week gestation period, horses have evolved to produce their young when the days are long and warm and the grass is green – the ideal environment for a growing animal and for a lactating mare with increased nutritional needs. However, we have interfered with nature’s design. With the creation of a universal birthday for Thoroughbred horses of January 1st and the economic demands to produce early foals for sale as mature yearlings, we have succeeded in altering the mare’s natural breeding season through use of artificial lighting programmes. A 200-watt light-bulb, in a 12-foot by 12-foot stall, switched on from dusk until roughly 11:00 pm nightly and beginning December 1st, is sufficient to advance the onset of the mares reproductive activity such that she should be ready to be bred by February 15th, the official start of the breeding season. From this, it is clear that light can control seasonal rhythms. What is more important in relation to jet lag is that light also closely regulates daily, or circadian rhythms. These circadian rhythms include changes in body temperature, hormone secretion, sleep/wake cycles, alertness and metabolism. A disruption of these rhythms results in jet lag.
It can be defined as a conflict between the new cycle of light and dark and the previously entrained programme of the internal clock. The first step to understanding jet lag is to examine the workings of the biological clock and the extent to which the daily cycles of light and dark can control physiological processes. All mammals possess a “master” circadian clock that resides in a specific area of the hypothalamic region of the brain. This area of the brain is responsible for regulating diverse physiological processes such as blood pressure, heart rate, wakefulness, hormone secretion, metabolism and body temperature. Each of these processes is in turn affected by time of day.
During daylight hours, the eye perceives light and light energy is transmitted via a network of nerve fibres to the brain. Here, the light signal activates a number of important genes and these “clock” genes are responsible for relaying signals conveying the time of day information to the rest of the body. Recent advances in the study of circadian rhythms and clock genes have shown that a molecular clock functions in almost all tissues and that the activities of possibly every cell in a given tissue are subject to the control of a clockwork mechanism. The role of the “master” clock in the brain is to communicate the light information to the clocks in the peripheral tissues, so that each tissue can use this information for its own purpose.
Thus, as day breaks and eyes perceive morning light, hormones are produced to help us to wake up, enzymes are activated in our digestive systems in anticipation of breakfast, heart rates increase, muscles prepare for exercise and many more circadian rhythms are initiated. This master clock in the brain, that controls so many bodily functions, must be reset on a daily basis by the photoperiod, whether it is sunrise and sunset or lights on and lights off, in order for an organism to be in harmony with its external environment. Jet lag occurs due to an abrupt change in the light-dark cycle and results from travel across multiple time zones, which in turn causes de-synchronization between an organism’s physiological processes and the environment.
Coupled to this is the fact that the circadian clock can only adapt to a new lighting schedule gradually and while the brain receives the light information directly, there is a further lag period involved in transmitting the time of day message to peripheral tissues. As a consequence, behavioural and physiological adaptation to changes in local time is delayed. This means that following a transmeridian journey, travellers are forced to rest at an incorrect phase of their circadian cycle, when they are physiologically entrained to be active and more importantly for the athlete, they are expected to perform when they are physiologically set to rest. As mammals, horses also suffer from the effects of jet lag.
Research is needed to understand the extent of physiological disruption caused by a transmeridian journey, the time period of the disruption and the overall effect it has on equine performance. Until now, no studies have been undertaken to investigate the physiological effects of jet lag in the horse. Studies in human athletes have demonstrated the detrimental effects of translocation on exercise capacity and performance. One early study examined human subjects following intercontinental flights consisting of eastward or westward journeys across multiple time zones (1). Results clearly demonstrated significant disturbances in heart rate, respiratory rate, body temperature, evaporative water loss and psychological function. Interestingly, these disturbances were found to be more profound following the eastward flight.
A more recent study conducted using top athletes from the German Olympic team investigated the effects of time-zone displacement on heart rate and blood pressure profiles (2). In athletes, blood pressure and oxygen supply to the organs are of utmost importance for optimal performance and successful competition. Rhythm disturbances in the 24-hour profiles of heart rate and blood pressure were found to be present up to day 11 after time-zone transition. The athletes were involved in intensive training programmes throughout the study and underwent frequent bouts of strenuous exercise.
Regular exercise at a set time in the 24-hour clock can strengthen circadian rhythms that are integral to physiological processes and can act as a timing cue secondary to light. It is also thought to aid in resynchronisation to a new time-zone. However, exercise was not found to improve the jet lag effects in this study, an observation that has relevance for the athletic horse in intensive training. The investigators concluded that following a flight across six time zones, athletes should arrive for their competition at least two weeks in advance in order to overcome the jet lag effects before competing.
Another study using fit human subjects examined performance times before and after an eastward journey across 6 time zones (3). Performance times for a 270m sprint were slower for the first 4 days following translocation as were times for a 2.8km run on the second and third days. This can be explained by the fact that the athletes’ internal body rhythms, including several neuromuscular, cardiovascular and metabolic variables and indices of aerobic capacity are out of synchrony with the environmental light-dark cycle following a transmeridian journey. Small mammals such as rats and mice have historically been used to study human circadian disorders such as jet lag. Current research being conducted at the Gluck Equine Research Center at the University of Kentucky has resulted in successful isolation of a number of ‘clock’ genes.
A comparison of these equine specific genes with their human counterparts has revealed an unusually high similarity between these two species at the DNA level, closer than the similarity observed between small mammals and humans. Unlike humans and horses, rats and mice are nocturnal animals and have yet to be proven as elite athletes. Further research is underway to investigate in detail the effects of jet lag on equine performance that will eventually lead to the development of measures to counteract these effects. Until then, information on the effects of transmeridian travel derived from studies on human performance can be used to provide guidelines to horse trainers, especially based on the similarity between the species in question. The severity of the jet lag effects can depend on a number of factors. These include the ability to preset the bodily rhythms prior to flying (4), the number of time zones crossed, the direction of the flight and individual variability.
Just as set exercise times can affect circadian rhythms in many physiological processes, feeding schedules also play an important role in entraining biological clocks, particularly within the digestive system. Horses anticipate feeding times. Banging of hooves on doors and rattling of empty feed buckets are common sounds that greet those responsible for feeding a yard of hungry horses. Therefore, it is important to change both feeding times and exercise schedules to mimic the new time zone prior to travel, in order to shorten the amount of time required for resynchronisation of digestive function and performance capacity upon arrival. Lighting is also of paramount importance. Exposing animals to early morning bright light for several days prior to an eastward journey across multiple time zones, or, extended hours of evening light prior to a westward journey, will help synchronize circadian rhythms to the new time zone prior to travel.
A recent study that tested a combination of approaches to hasten the resynchronisation of a group of elite sports competitors and their coaches to a westerly transmeridian flight, demonstrated the usefulness of combining melatonin treatment, an appropriate environmental light schedule and timely applied physical exercise to help the athletes overcome the consequences of jet lag (5). Melatonin, a hormone secreted by the pineal gland of the brain during the hours of darkness, is thought to help synchronize sleep-wake cycles and resynchronisation to a new time zone, but its suitability for these purposes has yet to be tested in the horse. Of course, these procedures to preset bodily rhythms need not be implemented if it is possible to arrive at the destination in sufficient time to allow natural re-entrainment to the new light-dark cycle.
For financial reasons, this is not necessarily feasible for the equine athlete. Two other important factors that determine the severity of jet lag effects are the number of time zones crossed and the direction of the flight. As one would expect, the greater the number of time zones traversed, the more severe the physiological disruption. For example, a flight from Europe to the East Coast of the United States, across six time-zones, would require a significantly greater resynchronisation time than a flight from the East Coast to the West Coast (three time zones), within the continental U.S. Any transmeridian journey in an eastward direction will result in a more profound disruption of circadian rhythms than a similar journey in a westward direction. The reason for this is a molecular one and involves the individual characteristics of certain clock genes. Suffice to say that clock genes react more rapidly to light than to darkness. When travelling in a westward direction i.e. from Europe to the United States, travellers enter an environment consisting of extended hours of evening light. The light continues to stimulate clock genes in the brain and adaptation to the new time zone occurs more rapidly. To some extent this may explain the success experienced by European horses at U.S. racetracks, even when they arrive three to four days prior to a race.
Knowing exactly how long it takes for the equine athlete to overcome any travel effects that may impinge on performance following such a flight, should provide valuable information to European trainers. In contrast, an eastward journey results in a shortened day length at the destination and requires a phase advance of the circadian clock. Travellers experience earlier nightfall and as the clock genes cannot respond well to darkness, an extended duration of jet lag. To emphasize, it will take an animal longer to adapt to the new light-dark cycle following an eastward flight and consequently longer to reach optimal performance levels following transit.
Pharmacokinetics deals with absorption, distribution, metabolism and elimination of drugs and these steps are influenced by physiological function of the body, which we now know to be influenced in turn by time of day. The implications of this for the athletic horse following transmeridian travel is worth highlighting, as it underlines the importance of knowing approximate physiological resynchronisation time to a new time zone. For example, terbutaline, a bronchodilator similar to clenbuterol commonly used by equine practitioners, has a significantly longer half-life when administered in the morning than in the evening (6). This implies that drug clearance times can be affected by transmeridian travel. In addition to the disruption of circadian rhythms, travel stress can also be a significant factor in further compounding the effects of jet lag following the transportation of horses across multiple time-zones.
Major complications associated with long-distance travel include pleuropneumonia, otherwise known as ‘shipping fever’, dehydration and colic. Even in cool conditions, horses will often lose 2-5 pounds of body weight for every hour they travel, as they do not like to drink while travelling (7). Care of horses during long-distance transportation is an extensive topic that requires separate attention. At the Gluck Equine Research Center, preparations are underway to conduct several experiments that will simulate phase advances and delays in the lighting schedule of groups of horses, thus mimicking eastward and westward journeys, so that the molecular and physiological effects of jet lag and the time duration of these effects can be investigated. The goal of this research is ultimately to provide practical guidelines to trainers in order that measures can be taken to counteract the detrimental effects of jet lag on performance, therefore leveling the playing field for horses competing away from home.