Sleep is often something we don’t get to see in horses, they are often working with us or eating (16-18 hour a day in the wild), but sleep disorders do occur. You are probably aware of insomnia, a disorder that humans can face that causes difficulty falling and staying asleep. This disorder can lead to sleep deprivation in humans, but were you aware that horses can also become sleep deprived?
Sleep is one of the most important things the body has to do alongside eating and drinking. Sleep contains two main phases called: REM (rapid eye movement), and NREM (non rapid eye movement). NREM is further split into 3 phases: N1, N2, and N3 (slow wave sleep). Horses have a fascinating mechanism called the “passive stay apparatus” which locks the stifle and knee joint in place allowing horses to move through the NREM phases while standing (I will refer to this style of sleep as dozing), however they must lie down for REM sleep. This is because REM sleep causes the muscles to relax completely and prevents standing. It has been described that horses will doze until they are ready for REM sleep, and then lay down. Horses have much shorter bouts of sleeping, typically 5 minutes of N3, followed by 5 minutes or REM, followed by 5 minutes of N3 which repeats several times during the day until the horse has completed around 30 minutes total of REM sleep.
Horses lie down in two main forms: lateral and sternal which can be seen below.
As you can see with the foal on the right, for REM sleep all muscle tension is gone and the weight of the head is on the ground.
If a horse has pain, is not comfortable with the environment, feels uncomfortable on the ground they have, or doesn’t have sufficient space, they may try to avoid lying down. This prevents them getting the much needed REM sleep. A horse cannot go very long without REM sleep, when they become sleep deprived, as mentioned above, while the horse dozes standing upright, rather than choosing to lie down they can literally fall into REM sleep. The video below could be distressing to some, however it shows footage from a research facility studying equine sleep deprivation and its effects of the physiological well being of the horse.
Sleep deprivation can be extremely harmful for horses, not only can they injure themselves when they fall (in the study the footage above is from they found over 90% of the horses had injuries after collapse in the knees (72.4%), fetlocks (68.4%), head (31.0%), or hocks (18.4%)), but the lack of (or traumatic nature of) sleep can result in reduced performance and more injuries during ridden work. The stress of collapse can be seen through stereotypical behaviour (colloquially called stable vices) as the above study also found 25% of the horses questioned displayed at least one stereotypy and 8 or the examined horses displayed one after their collapse, becoming listless and anxious. Recognising and diagnosing the causes of sleep deprivation are so incredibly important as 7 horses (out of 36) in the above study were euthanised 2 years post study for reasons related to the sleep deprivation. Sleep deprivation is hard to notice and diagnose as owners often won't see horses overnight, and night vision cameras are expensive. There are however some key features to watch out for:
Injuries to the areas mentioned above
Sleepiness during the day, or lethargy
Collapse during the day or night
Inappropriate sleeping or refusal to get up during tasks
Bedding that is irregularly disturbed
If you have concerns about your horses' sleep, see if you can monitor their lying behaviour, speak to your vet about it and make management changes to make them more comfortable. Some general suggestions are below:
Deeper bed
Group turnout with friends where there is no bullying
Larger stable
Key references:
The main sleep deprivation study in horses by Fuchs et al., (2018): https://equitationscience.com/media/the-dangers-of-sleep-deprivation-in-horses
An interesting thesis discussing equine sleep and the impact of light and bedding by Amiouny, (2020): https://bsas.org.uk/assets/uploads/docs/entries/DAmiouny_Equine_Sleep_Project.pdf
A key paper on equine sleep by Aleman, Williams, and Holliday, (2008): https://aaep.org/sites/default/files/issues/proceedings-08proceedings-z9100108000180.pdf
An interesting summary of different diagnoses of collapse by Keen, (2015): https://www.magonlinelibrary.com/doi/abs/10.12968/live.2015.20.3.166
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