Sleep Deprivation in Horses, Part 1 of 2

Common sleep patterns and needs
by Dave Sauter, DVM

We all need our sleep to be healthy, and we need to be healthy to sleep. That is as true for horses as it is for us. In this two-part series we explore the subject of sleep in horses and what happens if they don’t get enough of it.

All of us sleep, usually every day. We know how it feels if we don’t get enough. Yet for all our experience, there is much about the subject of sleep that remains a mystery. Sleep has attracted the attention of medical minds since the days of Hippocrates in 400 B.C. It wasn’t until the 19th century, however, that science began to take a serious look at the subject. As our understanding of sleep has grown, so has our recognition of its importance to our health. In 1996 the American Medical Association recognized sleep medicine as a specialty. This subject has also received more attention in equine medicine in the last several years.

What is Sleep?
How is sleep different from other forms of unconsciousness, such as a coma? Sleep is a state of immobility with greatly reduced responsiveness. Unlike hibernation, sleep is rapidly reversible. It is possible to quickly wake up to a state of nearly complete mobility and mental alertness. During sleep there is unconscious sensory input and processing. Hence a crying baby may wake a worried parent while a thunderstorm might not. When sleep is prevented, there is an increased desire and need for it.

Sleep can be divided into two types, Rapid Eye Movement (REM) sleep and Non-REM (NREM) sleep. REM sleep is characterized by brain wave patterns, heart rate and respiratory rate similar to when awake, decreased muscle tone, and, as the name states, rapid eye movements. REM and NREM sleep have been identified in birds and all of the 60 – 70 terrestrial mammals that have had sleep studies. Marine mammals sleep with half of their brains at a time (and keep one eye open and one closed).

Amongst herbivores, the larger the species, the less time is spent sleeping. In general, carnivores sleep more than omnivores and omnivores more than herbivores. Animals that are born more immature tend to need more REM sleep. Here are a few examples (in hours of sleep needed per day):
• Cat - 12
• Dog - 10
• Human - 8
• Goat - 5
• Cow, elephant, sheep - 4
• Donkey and horse - 3
• Giraffe - 2

Sleep Patterns in Horses
Horses require much less sleep than their human counterparts, only three to five hours per day. Drowsiness and the equivalent to a human's lighter planes of sleep can be achieved while standing. Horses take on a characteristic stance when engaging in this level of sleep. Weight is supported on both front legs. On the hind limbs, one leg supports weight while the other is unloaded. Some speculate that this ensures one hind limb is in the “cocked and ready” position in the event a threat approaches while horses are resting. Sleep studies suggest that the hindquarters are pointed toward the greatest perceived threat (e.g., the stall door). This drowsy posture can progress into Slow Wave Sleep (SWS), equivalent to the deeper planes of sleep in people. Unlike people, horses can achieve SWS while on their feet. Their heads will characteristically drop to a lower level. They may, however, decide to lie down to continue their SWS in sternal recumbency (i.e., on their chests).

Recall that in REM sleep, muscles have greatly reduced tone. This is true for the horse as well. This means that for horses to enter REM sleep they must be recumbent (lying down). Generally, horses in REM sleep will lie flat out (lateral recumbency), but occasionally they will remain in sternal recumbency, bracing their chins on the ground passively as the poll drops when muscle tone decreases. Horses only require about 30 minutes per day of REM sleep, much less than people, who average about two hours per day. Outward features of REM sleep in horses include paddling, twitching, blinking, rapid eye movement, ear twitching and flaring of the nostrils.

Sleep deprivation will occur in horses if they are either prevented from lying down or unwilling to do so. Next month, we will look at the symptoms and specific causes of equine sleep deprivation, and how they relate to your horse's overall health.