Short days and a lack of sun can cause mood swings and low energy in humans, but do horses suffer in the same way? Gil Riley MRCVS sheds some light.
If you tend to feel low and lacklustre at this time of year, you’re not alone. An estimated 8-10% of people living in northern regions such as the UK and Scandinavia suffer with seasonal affective disorder (SAD).
It is a recognised medical condition that brings about depressive symptoms and lack of energy in winter.
But can horses really suffer from SAD? Some owners believe that their animals are more lethargic in the winter months and less enthusiastic during ridden work. No doubt some of these horses are experiencing pain from low-grade arthritis in the joints, exacerbated by the colder weather, or from ulcers in the stomach irritated by an increased amount of hay in the winter diet.
Other elderly equines may be suffering from Cushing’s disease, a recognised cause of lethargy in the winter months. But in horses where these – and other – options have been eliminated, could it be that they are actually suffering from the winter blues?
Rhythms of life
Light is the most important environmental factor that controls the body’s rhythms. Its effect on animals that breed seasonally, such as the horse, is clear.
Along with the photoreceptor cells responsible for vision, the equine eye contains light-sensitive cells that transmit information via the optic nerve to the suprachiasmatic nuclei within the brain.
These nuclei lie within the hypothalamus and are more commonly referred to as the home of the “circadian clock”. This structure oversees body rhythms, partly by determining the release of melatonin – a hormone that controls sleep patterns.
The variations in physiology and behaviour exhibited by both horses and humans throughout any 24hr period are known as circadian rhythms. While we humans are diurnal creatures – most active during daylight hours – horses are crepuscular, meaning they are more active at dawn and dusk.
SAD in humans is clearly linked to disruption of these circadian rhythms, as winter approaches and the nights draw in. Yet rather than being able to follow their natural circadian rhythms, as they would in the wild, our horses are generally subjected to a human-driven routine that may well be disrupted further by seasonal variation.
Additionally, prolonged periods of dark lead to increased levels of sleep-inducing melatonin. In humans, this triggers a reduction in serotonin — a lack of which can cause depression.
While we don’t know for sure that the exact same process occurs in horses, it would seem logical to suggest that it might, and that this biochemical imbalance could induce similar SAD-type symptoms.
Blind trials
Light therapy, where artificial light is used to counteract darkness during winter months, has proved useful in alleviating SAD in humans.
Relatively little is known about the effects of light therapy on the well-being and behaviour of horses, but a pathfinding study of stabled horses led by Dr Carol Hall at Nottingham Trent University produced some illuminating results.
Trial horses were placed under broad-spectrum, high-intensity light, chosen to mimic daylight, for an hour every day for six weeks. Their behaviour before and after the trials was then compared with similar horses who had not received light therapy.
Blind trials were conducted so that the assessors recording the behaviour of the horses did not know which of them had been treated. The criteria recorded were feeding behaviour, sleep patterns and the horses’ attitude to being handled and ridden.
Although this was only a small-scale preliminary study, differences were noted between those horses exposed to the light and those who weren’t.
Those who had received light treatment showed no significant variation before or after light exposure in any of the areas measured in the study.
But the untreated horses were found to be sleeping for longer and were described as lazier at ridden exercise at the end of the six weeks. The yard manager also reported that the light-treated group were less grumpy.
Let there be light
The results suggested that keeping horses stabled for long periods — and thus exposing them to longer periods of low light or darkness — may well have the effect of producing SAD-like symptoms at any time of year.
Indeed, studies in the USA have shown that if a stabled horse is taught how to turn the stable light on, he will. But there’s an important factor to consider before using light therapy.
The equine eye is adapted for low light levels and their vision is not as good as ours in brightly lit conditions. As a prey animal, a horse’s eyes see best at dawn and dusk when its natural predators are most active.
While this does not mean that a horse needs less light than we do in terms of hours per day, it does mean that strong lighting has the potential to cause discomfort.
Dr Hall’s study, in conjunction with our proven knowledge of how light affects the horse in other ways, would lend support to the idea that horses should receive light for a generous part of each day. Keeping them in darker environments for prolonged periods may not only represent poor husbandry in terms of quality of life and socialisation with others, but could — in some animals — lead to SAD.
If depression is suspected and your vet has ruled out obvious physical ailments, exposure to more light may be a good place to start. While much work remains to be done, early studies and anecdotal evidence suggest that light therapy may be a useful tool for gaining optimum equine performance year round.
Light and the breeding cycle
The natural reproductive period of the horse coincides with the light-filled days between May and September, when the environment is optimal for the survival of offspring.
When these high light levels are detected by the retina at the back of the eyeball, melatonin production decreases – which in turn sends a message to the hypothalamus that it is time for the reproductive process to begin.
The hypothalamus then produces gonadotropin-releasing hormone (GnRH), the secretion of which stimulates a small gland found at the base of the brain, the pituitary, which in turn secretes wo key reproductive hormones – luteinizing hormone (LH) and follicle stimulating hormone (FSH).
These are carried via the bloodstream to the ovaries of the mare and the testes of the stallion.
Nature dictates that the mare should be receptive to the stallion in late spring or early summer, in order to produce a foal — 11 months later — when the grass is green and the weather is warm.
Through the use of electric light, however, we can extend the light that the mare is exposed to daily and thus trick her into cycling earlier. This method is most widely used in the thoroughbred industry, where early foals have a developmental edge when they begin racing at two.
Light and jet lag — is there a link?
Jet lag occurs due to an abrupt change in the light-dark cycle resulting from travelling acrossmultiple time zones. This in turn causes desynchronisation between an animal’s physiological processes and the environment.
Each of the processes regulated by the horse’s body clock, including blood pressure, heart rate, temperature, wakefulness and metabolism, is affected. Severity depends on the number of time zones crossed and flight direction.
But research suggests that horses are rather resilient to jet lag (vet news, 16 August 2012) and are able to reset their body clocks faster than humans or rats. Bizarrely, it seems that performance can even be boosted by travel, thanks to an increase in levels of a hormone called prolactin in the horse’s body.
So the jury is out on whether exposing horses to light before and after flying makes any long-term difference in jet lag cases — and whether it even matters.