Resting and Sleeping
Last revised April 16, 2017.
Sleeping with One Eye Open. Paradoxical sleep, lateral recumbency.1 Lateral recumbency is believed to be brief because of the respiratory stress it places on the horse.
We all need to sleep. Every living thing seems to sleep (or have a state of suspended animation of some sort), including plants, corals, snails, insects, fish, reptiles, birds and mammals. In deep sleep in ants, queens and workers fold their antennae, become non-responsive to contact with other ants, and may show rapid antennal movement (RAM sleep), their equivalent to our REM (rapid eye movement) sleep.2 Dreaming accompanies REM sleep, and it is reasonable to assume that ants are dreaming during RAM sleep. Electric eels dream, suddenly discharging when asleep.3 For us, sleep provides biological, physiological, and psychological restoration.4 We must assume that it does the same for horses, ants, and electric eels.
A horse divides his day into alternating periods of wakefulness and sleep. Sleep is composed of both Slow Wave Sleep (SWS) and Paradoxical Sleep (PS).
During SWS, brain waves are slow and regular, and the horse is not dreaming. A horse can engage in SWS both standing and in the sternal recumbent position. In this state, the horse’s head is down, and his lower lip has relaxed. Eye movement is diminished, and the eyelids may be open a bit. There is usually a small amount of muscular activity, and while the horse’s vigilance is reduced, the horse can still respond quickly to disturbing sounds or smells. This is sometimes called “the sleep of the brain” because, in the absence of dreaming, many parts of the brain have become inactive.
When standing in slow wave sleep, one hind leg is usually flexed, which gives that joint a bit of rest and locks the other hind leg into a fixed position that holds the horse up with no muscular effort — the stay apparatus.5
Palomino in slow wave sleep while waiting for a riding lesson.6
If a horse is very relaxed and confident of its safety, it may become sternally recumbent in slow wave sleep. In this position, all leg joints may be bent. Up at the front, the horse is vertical, with sternum on the ground. But toward the back, the horse rests on one hip, with the other more vertical.
Sternally recumbent American Cream draft horse sleeping.7
During paradoxical sleep — AKA activated sleep, deep sleep, desynchronized sleep, rapid eye-movement or REM sleep, para sleep, and rhombencephalic sleep — the horse lies on its side (lateral recumbency), his muscles completely relaxed. While eyelids are closed, the eyes move rapidly around, in Rapid Eye Movement. This is deep sleep or REM sleep — sometimes called “the sleep of the body” — and a time of intense dreaming. In lateral recumbency, the legs closest to the ground are usually pulled in a bit farther than the top legs, and legs are likely bent at all joints. During lateral recumbency, the horse may roll, pausing briefly on his back (at the peak of “dorsal recumbency”), which allows abdominal viscera to reposition themselves. A rapid roll can cause a gut twist. So best to let a sleeping horse lie.
Tying a horse in a stall, or otherwise preventing a horse from becoming laterally recumbent, is not a good thing. A horse must be lying on its side to engage in REM sleep, and without this opportunity, the horse quickly becomes sleep deprived.8 The palomino in an earlier picture might have preferred to wait for his next rider while sleeping on his side. Foals require much more sleep than adult horses, particularly deep sleep, so when transporting foals, be sure to give them space to lie down.
A crowded paddock also seems to prevent lateral recumbency and thus REM sleep,9 perhaps because of the increased commotion, or perhaps because of the risk of being stepped on by another horse.
Horses sleep fewer hours each day than their predators do, and even fewer hours than burrowing prey animals, such as rabbits. Sleep can be a dangerous thing. So horses sleep more when they feel comfortable, in a familiar place.10 They sleep more when a group leader is sleeping,11 and a group leader is usually the first to lie down to sleep.12 [I want to delete this last sentence, having read my chapter on Leadership. But that’s what these researchers said.]
Horses don’t sleep once a day for a long block of time, as do humans, or several times a day, in shorter blocks, as do young and old humans. Instead, they may sleep for less than 15 minutes per hour, with each sleep consisting of a 5 minute slow wave sleep, then a partially awake moment when he checks for danger and transitions to lateral recumbency, then a 5 minute paradoxical sleep, and finally another 5 minute slow wave sleep. After this 15 minutes of sleep, the horse may be awake and active for about 45 minutes before resuming this sleep cycle. Total sleep time per day ranges from 2.5 hours to 5 hours.13 Horses seem to do more sleeping at night than during the day, and especially are likely to do more deep sleep in a fully recumbent position at night.
Drowsiness is likely a form of sleep, and though it is inefficient in producing the benefits of sleep, it is safer to be drowsy than sleeping. Horses in a pasture sleep less than when in the barn, but show more drowsiness.14
Sleeping with One Eye Open
Birds and aquatic mammals are the only taxonomic groups known to exhibit unihemispheric slow-wave (deep or non-REM) sleep — sleep on one side of the brain at a time. How can swifts fly non-stop for months15? I believe that this can be accomplished by sleeping on one side of their brain, then on the other. When in flight, a bird sleeping on just one side of their brain still has another hemisphere to handle the details of flight, and even though it appears that sleeping birds do a lot of gliding and soaring, flight is tricky business. At a roost, a chicken often engages in unihemispheric sleep, with the open eye generally facing toward the greatest source of danger. When four chickens are lined up on a perch, those on the ends of the line are likely to direct their open eye away from the group.16
As I write this, I don’t know if horses ever fully close both eyes at the same time when sleeping. All of the horses I’ve seen sleeping seem to have at least one eye partly open, even when dreaming.17 Search YouTube for “horse sleeping” and see if you can find any.
I think it is possible that one side of a horse’s brain can be sleeping, while the other side is awake — that they can exhibit unihemispheric slow-wave sleep. This would be extremely valuable for horses in the wild, who regularly sleep while standing.
Fly Defense During Sleep
If you look at the picture below, the middle horse is doing much of the work with his tail. The other horses bring their heads to it, to benefit from its fly-swatting powers. In exchange in this group, the horse with its rump to us gets two assistants to swat flies.
Resting behavior in a band of feral horses is often characterized by huddling. Aside from the physical protection received by this behavior, the head-to-tail orientation of band members in this type of resting behavior also facilitates grooming. This allows for insect control around the head by the tail-swishing effect of neighbors.18
1 Photo source: “The Cutetest Loud Snoring Horses ever” https://www.youtube.com/watch?v=w90nF1x2Rko
2 Cassill, D. L., Brown, S., Swick, D., & Yanev, G. (2009). Polyphasic wake/sleep episodes in the fire ant, Solenopsis invicta. Journal of insect behavior, 22(4), 313-323.
3 Montgomery, Sy. The soul of an octopus: A surprising exploration into the wonder of consciousness. Simon and Schuster, 2015. p. 37.
4 Houpt, K. A. “The characteristics of equine sleep [Horses, behavior].”Equine Practice (1980).
5 Sack, W. O. “The stay-apparatus of the horse’s hindlimb: explained.” Equine practice (USA) (1989).
6 Image source: https://commons.wikimedia.org/wiki/File:Nap_time.jpg
7 Image source: https://upload.wikimedia.org/wikipedia/commons/f/f1/Sleeping_American_Cream_Draft_Horse.jpg
8 Hale, L. A., and S. E. Huggins. “The electroencephalogram of the normal “grade” pony in sleep and wakefulness.” Comparative Biochemistry and Physiology Part A: Physiology 66.2 (1980): 251-257.; Dallaire, A., & Ruckebusch, Y. (1974). Sleep and wakefulness in the housed pony under different dietary conditions. Canadian Journal of Comparative Medicine, 38(1), 65.
9 Glade, M. J. “Social sleeping behavior in young horses.” Equine Practice (USA) (1984).; Glade, Michael J. ““Social sleeping” among confined horses.” Journal of Equine Veterinary Science 6.3 (1986): 156-157.
10 Ruckebusch, Y. “Comparative aspects of sleep and wakefulness in farm animals.” The Sleeping Brain, Perspectives in the Brain Sciences 1 (1972): 23-28.
11 Hendricks, J. C., and A. R. Morrison. “Normal and abnormal sleep in mammals.” Journal of the American Veterinary Medical Association 178.2 (1981): 121-126.
12 Houpt, K. A. “The characteristics of equine sleep [Horses, behavior].”Equine Practice (1980).
13 Belling Jr, Theodore H. “Sleep patterns in the horse.” Equine Practice 12 (1990): 2-26.
14 Houpt, K. A. “The characteristics of equine sleep [Horses, behavior].”Equine Practice (1980).
15 Lockley, R. M. (1969). Non-stop flight and migration in the common swift Apus apus. Ostrich, 40(S1), 265-269.; Liechti, F., Witvliet, W., Weber, R., & Bächler, E. (2013). First evidence of a 200-day non-stop flight in a bird. Nature Communications, 4.
16 Rattenborg, N. C., Lima, S. L., & Amlaner, C. J. (1999). Facultative control of avian unihemispheric sleep under the risk of predation. Behavioural brain research, 105(2), 163-172.
17 For example, see “My horse running in her sleep” https://www.youtube.com/watch?v=hymzSlzmvGE
18 image source: Ransom, Jason I., and Brian S. Cade. “Quantifying Equid Behavior–A Research Ethogram for Free-Roaming Feral Horses.” U.S. Geological Survey Techniques and Methods 2-A9, 23 p. (2009) http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1018&context=usgspubs