What happens when your horse sleeps? Does he dream like us, or is it just a period of blankness and inactivity?
The need to sleep is one of the most insistent drivers in the body, outside of breathing or reaction to pin. People can go on hunger strike but ultimately, they cannot stop themselves sleeping, except for relatively short periods of time.
And depriving people and animals of sleep impairs both learning and ultimately, health, as has been shown in sleep deprivation research, and in attempts to torture people.
What’s it for?
So why do humans, mammals, birds and even reptiles, all do it?
To understand why we sleep, first, we should appreciate that sleep is a behaviour That may sound strange, because we are more used to associating activities such as walking or eating with behaviours. But sleep is such a mixture of activity and rest that, although we often liken it to the state of unconsciousness, this is not an accurate description of it.
It is certainly true to say there is a change in consciousness when we sleep but we are not unconscious. Interestingly, although there are some very particular aspects in the patterns and frequency of sleep in horses, when they do sleep, they do so in a manner that is remarkably similar to humans. So what really happens to your horse when he sleeps?
Electricity and chemistry!
The upper brain stem is the part of the brain that is most instrumental in driving whether we or horses wake up or fall asleep. it is the ‘pacemaker’ of the sleep-wake cycle.
Through electrical impulses, it promotes activity in the forebrain and as it does, we are roused from sleep. In experiments performed on rats, where part of the brain stem was severed, the forebrain was not activated and the rats simply remained asleep all the time.
There is clear evidence that there are also very specific chemical messengers (or neurons) that work in conjunction with the electrical impulses that control our activities. One of these chemical messengers, known as serotonin, has a direct effect on the length of time we sleep, and we now know that low levels of serotonin mean less than normal sleep times.
Animals and humans who suffer from depression usually show abnormal sleep patterns (as a result of low serotonin levels), and significant time and study is now being devoted to attempting to improve sleep behaviours as a way of controlling depression.
Is it just for tired animals?
Why we sleep is the subject of continuing research. A number of scientists initially focused on the idea that sleep is a process of restoration and regeneration, but under close scrutiny, this proved to be an incomplete assessment. if all the brain was doing was helping the body recover from the stresses and strains of the day, then surely if we had run a marathon during the day, for example, we would require three or four times as much sleep as usual?
Even people who have been deprived of sleep for several days usually require only an hour or two extra the next night after to recover. Certainly they do not need to regain the 20 or 30 hours of sleep they have lost. Equally, people who have spent almost the entire day dozing on the beach, still sleep at night.
The idea of sleep being necessary only when an animal is tired is thrown into further disarray when we look at many marine mammals, such as the bottle-nosed dolphin or the porpoise. They have developed an extraordinary pattern of sleep in which one hemisphere in the brain remains alert in wakefulness whilst the other part of the brain sleeps. Not only is this essential for dolphins to allow them to continue to come to the surface to breathe, presumably they have also found advantages in being able to remain on the alert for predators but still get the necessary sleep.
Nothing to do?
Other scientists have suggested that sleep has evolved to keep an animal safe when there is nothing to do at night time. For humans this could perhaps be a plausible theory. Our ancestors would have done their hunting by day when it was easier to see and could have then learned to lie down in a safe place and not waste energy at night. Similarly, nocturnal animals that are active at night, spend most of the day time sleeping. But the theory also starts to unravel when we look at the many animals that acquire their food during day and night and therefore sleep whenever they can, including when they have the option to be active.
Surely for these animals, sleep would have been simply eliminated through the millions of years of evolution as an unnecessary interruption?
Part of the answer to this lies in the fact that sleep must be much more than just a response to a need of tiredness or to environmental changes at dawn or dusk, and its presence in all mammals and birds suggests that it must be necessary for some other reason.
The Stages Of Sleep
Studies of both human and animal subjects at sleep laboratories have shown that sleep is divided into two types, and Stage 3 and 4 of the first of these is called Slow Wave Sleep (SWS).
SLEEP TYPE 1
This is actually a transition between wakefulness and sleep as the animal becomes drowsy, starts to lower his head and close his eyes. it is the lightest form of sleep and sometimes is fleeting in duration.
Scientists believe this is the time when the brain uses electrical activity to ensure that the animal stays asleep. Older people, for example, do not show the same level of activity in Stage 2 sleep and as a result many of them report that they wake off and on throughout the night.
SLOW WAVE SLEEP
The animal starts to sleep soundly (even snores) and the body stops moving.
SLEEP TYPE 2 RAPlD EYE MOVEMENT (REM) SLEEP
After Stage 4, an animal moves into the second type of sleep known as REM or rapid eye movement sleep. Now a loss of muscle tone is accompanied with rapid movement of the eyes as they dart back and forth under closed eyelids (we now know that apart from breathing and eye movements, the body of the animal is actually paralysed during REM sleep). The heart rate goes up and down, breathing can be irregular and brain scans show that the brain becomes more active. in many respects, there are similar aspects to REM sleep and being awake. Scientists now think that REM sleep is the time both humans and animals practice becoming more sensitive to stimuli in the environment.
Certainly for horses, it helps them be more vigilant by becoming more sensitive to danger and so they become better at escaping from predators REM sleep also has an important effect on the learning centres of the brain. It seems to be the time when the brain can take stock of the experiences of the day by either; consolidating the information it needs, or flushing out unwanted, useless things that would just clutter up storage space.
In addition, this kind of sleep is critical to the brain’s ability to sort out and integrate differences between learned events and instinctive behaviours, such as a horse’s learned responses to fear-inducing events like being loaded into a trailer, which are coupled with instinctive fears.
REM sleepis also called paradoxical sleep because when it was first discovered in 1953, the scientists believed there was an unexpected paradox between the fast movements of the eyes and the stillness of the rest of the body.