In our Ask the Vet column, Dr. Lydia Gray answers your horse-health questions at HorseChannel.com/AskTheVet. Got a question for Dr. Gray? Send it to firstname.lastname@example.org and use subject line “Ask the Vet.”
Q: My Quarter horse mare has developed a strange behavior. Suddenly she will act drowsy, eyes half closed, head slightly down. Then she rocks back and forth like she’s trying to keep her balance. Sometimes she’ll go down on her legs, like a horse would when about to lie down; then she’ll stand up again. When touched anywhere around the chest, throat or the underside of her stomach she bites out, which is very unusual for her. It really scares us! The first time was really bad. We called a vet, but by the time he got there she was acting fine. He looked her over and found nothing wrong. She’s done it two times since. Within about 20 minutes, she’s just fine. What could be wrong with my girl?
Just like humans, horses need to be able to get sufficient REM sleep or they will start so show signs of sleep deprivation.
A: There could be a number of explanations for the behavior you describe, such as true narcolepsy, HYPP, heart disease, epilepsy, EPM, and others, so I encourage you to keep working with your veterinarian to try and get an accurate diagnosis. You may find that keeping a daily journal will uncover a pattern that leads to an answer.
Hopefully though, your mare has nothing more serious than sleep deprivation. While we’ve all heard that horses sleep standing up, the truth is that even horses need between 30 and 60 minutes of REM sleep each day. After 7 to 14 days of being deprived of this type of sleep, horses will begin to have “sleep attacks” such as you describe.
REM sleep in horses can only be achieved when lying down, not standing, so sleep-deprived horses either can’t or won’t lie down. There are both physical and mental reasons for not lying down. If you never see your horse roll, or never see mud or shavings on her, she may not be physically able to get down and back up. This would be another reason to have your vet back out to examine your horse, to see if there are any musculoskeletal or other reasons for pain. Once the pain is controlled, these horses may be able to lie down again and get REM sleep.
There are several mental reasons why horses won’t lie down as well. One is the absence of what’s called a “sentinel” or guard horse. Horses that suddenly find themselves alone or that have lost the dominant horse in a herd can become sleep-deprived because they do not feel they can safely lie down and sleep soundly. Adding a companion—whether a watch horse or even a goat—usually resolves these types of situations.
Sometimes the environment itself is just not conducive to deep sleep. In one situation, a horse was taken to the county fair for a show and started collapsing in the cross ties. With a little detective work, it was discovered that, in addition to all the (ab)normal sounds of a county fair, there was a fireworks display every night! This poor horse hadn’t slept in over a week! When the owners took him home, he slept straight through one whole day and never collapsed again.
FYI what separate sleep attacks from true narcolepsy is that sleep-deprived horses go through the first two stages of sleep—a deep restfulness phase and a slow wave sleep—before entering REM sleep and partially collapsing. Narcoleptic horses go from full wakefulness to full sleep (and vice versa) immediately, with no drowsiness or other sleep phases in between.