Zubair Khalid

Virologist/Molecular Biologist | Veterinarian | Bioinformatician

Conventional & Molecular Virology • Vaccine Development • Computational Biology

Dr. Zubair Khalid is a veterinarian and virologist specializing in conventional and molecular virology, vaccine development, and computational biology. Dedicated to advancing animal health through innovative research and multi-omics approaches.

Dr. Zubair Khalid - Veterinarian, Virologist, and Vaccine Development Researcher specializing in Computational Biology, Multi-omics, Animal Health, and Infectious Disease Research

Category: Blog

Sleeping Standing Up: How Some Animals Do It

Horses, elephants, and many bird species can sleep while standing, but this ability comes with an important limitation. Standing sleep is generally light sleep. The deeper rapid eye movement (REM) sleep that is essential for memory and brain restoration requires lying down in most large mammals. This article explains the anatomical adaptations that make standing sleep possible, identifies which animals use this strategy, and describes why farmers and animal caretakers should monitor lying behavior as a welfare indicator.

At a Glance

Animal Group Can Sleep Standing Requires Lying Down for REM Sleep Key Adaptation Management Implication
Horses (Equus caballus) Yes Yes Stay apparatus in legs allows joint locking Horses that never lie down may be REM deprived
African elephants (Loxodonta africana) Yes Yes Columnar limbs with passive weight support Adults may stand for long periods but need recumbency for deep sleep
Birds (multiple species) Yes Variable Tensegrity system of bones and tendons Perching birds can rest standing without muscular effort
Walruses (Odobenidae rosmarus) Yes Yes Can sleep floating, on bottom, or standing in water REM sleep in water occurs during single long apneas
Hippopotamuses (Hippopotamus amphibius) Yes Yes Can rest standing on bottom with nostrils above water Rest occurs in water and on land with species-specific patterns

What Standing Sleep Means Biologically

Sleep in animals is defined by behavioral criteria including reduced responsiveness, typical posture, and rapid reversibility. The fruit fly Drosophila melanogaster meets these behavioral criteria for sleep and shows pharmacological and molecular correlates of mammalian sleep, which suggests that sleep-like states are ancient and widespread across the animal kingdom. The function of sleep remains a long-standing mystery in neurobiology, but its conservation across species indicates fundamental biological importance.

For large herbivores, the ability to sleep standing offers a survival advantage. A horse or elephant that can rest without fully collapsing to the ground remains ready to flee predators. However, this advantage comes with a tradeoff. The muscle relaxation that accompanies REM sleep would cause a standing animal to fall. Therefore, animals that sleep standing must either limit REM sleep to brief episodes or find a posture that supports their body weight without active muscle tension.

The Stay Apparatus in Horses

Horses possess a specialized system of tendons and ligaments called the stay apparatus that allows them to lock their major joints and stand with minimal muscular effort. This system engages the forelimb and hindlimb, permitting the horse to doze while standing. The stay apparatus is not a single structure but a coordinated arrangement of passive support across the shoulder, elbow, carpus, and fetlock joints.

The practical consequence for horse owners is that a horse standing quietly with a relaxed lower lip, drooping ears, and a resting hind leg is likely in light sleep. This state is normal and should not be mistaken for illness or distress. However, the stay apparatus does not permit REM sleep. During REM sleep, muscle tone is actively suppressed, and the horse must lie down to avoid collapsing.

Research on stabled riding school horses found that lighting conditions influenced total sleep, recumbency, wakefulness, and standing sleep, with the lighting period affecting these behaviors in both treatment and control conditions. This finding indicates that standing sleep is a measurable and distinct behavioral state that responds to environmental management.

Why REM Sleep Requires Lying Down

REM sleep is characterized by rapid eye movements, muscle atonia, and irregular breathing. In horses, REM sleep is associated with learning and memory consolidation. A field study of 16 healthy horses found that longer REM sleep was associated with a greater number of reversals in a spatial learning test, while no relationship was found with error rate. Horses with shorter REM sleep had a 50% chance of progressing after five reversals compared to six reversals for longer REM sleepers. This association between REM duration and learning performance suggests that adequate REM sleep supports cognitive function.

The muscle atonia of REM sleep means that a standing animal would lose postural support. Horses therefore lie in sternal or lateral recumbency for REM sleep. Lateral recumbency, where the horse lies flat on its side, is the typical position for REM sleep in ungulates. A study of nocturnal behavior in 192 individuals of 18 ungulate species from 20 European zoos found that most species showed a significant increase in the proportion of lying during the night, and the time between two events of lying down showed a high degree of rhythmicity in all species.

Tensegrity and Standing Sleep in Birds

Birds can rest and even sleep standing up because of a biomechanical system called tensegrity. In this system, rigid bones are held together by tension in tendons, allowing the system to stabilize under the action of gravity. A mathematical model of the bird leg found that a single cable representing the extensor muscles and tendons could not achieve stable balance, but replacing the single cable with four cables corresponding to the extensor groups and adding a ligament loop at the knee allowed the model to reach stable equilibrium.

This tensegrity arrangement is an innovation unique to the avian lineage and is fundamental to the evolution of the bird body plan. For poultry farmers and bird keepers, this means that birds observed resting on one leg or standing with eyes closed are likely sleeping or resting without expending muscular energy. The passive stability of the bird leg means that standing rest does not fatigue the bird.

Elephants and the Limits of Standing Sleep

African elephants can sleep standing, but they also require recumbency for REM sleep. A study of nocturnal behavior in African elephants found that lying behavior increased during the night, consistent with the pattern observed across ungulate species. The proportion of lying in a given period was greater in Artiodactyla than in Perissodactyla, which includes horses and elephants, and greater in juveniles than in adults.

For elephant caretakers, the practical implication is that adult elephants may stand for extended periods, but they still need to lie down for deep sleep. An elephant that never lies down may be experiencing pain, discomfort, or environmental stress that prevents recumbency. Monitoring lying behavior is therefore a useful welfare indicator.

Walruses and Aquatic Sleep

Walruses demonstrate that standing sleep is not limited to terrestrial animals. A study of four captive walruses videotaped continuously for 7 to 17 days found that walruses slept while floating at the surface, lying on the bottom, or standing and leaning against the pool wall. REM sleep in water occurred in all positions. When in water, walruses were predominantly awake 88 to 99% of the time, but on land they were asleep on average 40 to 74% of the time.

The breathing patterns of walruses during sleep are remarkable. On land, breathing was regular during quiet sleep with most pauses under 30 seconds, and arrhythmic during REM sleep with apneas lasting up to 160 seconds. While in water, irregularity increased further with apneas over 4 minutes, and all REM sleep episodes occurred during a single apnea. This adaptation allows walruses to sleep in water without drowning.

Hippopotamuses and Semi-Aquatic Rest

Hippopotamuses rest both on land and in water. A study of an adult pair and their two-month-old calf at a zoo found that during the day the animals spent equal time on land and in water, but at night they spent on average 76% of their time in water. Rest occupied 48 to 53% of the 24-hour period. On land, the animals rested in a lying position. In water, they rested lying, sitting, or standing on the bottom with nostrils, eyes, and ears above the surface, or submerged and lying on the bottom.

All displayed eye jerks and twitches characteristic of REM sleep. On land, 98% of breathing pauses lasted under 30 seconds. In water, pauses ranged from 4 to 145 seconds, with most lasting under 1 minute. The female's sleep was more fragmented, less deep, and less vigilant due to calf care, which demonstrates that maternal responsibilities affect sleep quality in hippopotamuses as they do in many mammals.

Ungulate Nocturnal Behavior Patterns

The study of 18 ungulate species in European zoos provides a broad picture of how hoofed animals distribute their rest across the night. Differences between individuals of different age were found, but no differences with respect to sex were seen. Most species showed a significant increase in the proportion of lying during the night. The proportion of lying in a period was greater in Artiodactyla than in Perissodactyla, and greater in juveniles than in adults.

For farmers managing cattle, sheep, goats, and other artiodactyls, this means that nighttime lying is the norm. Animals that remain standing through the night may be experiencing discomfort, lameness, or inadequate bedding. A follow-up study modeling coordination and regularity in nocturnal behavior of ungulates in zoos further examined these patterns, confirming that lying behavior follows regular rhythms.

Practical Assessment of Standing Sleep in Farm Animals

Farmers and animal caretakers can assess whether their animals are getting adequate sleep by observing behavior at regular intervals. The following steps provide a practical approach.

Step 1: Establish a Baseline Observation Schedule

Observe animals at multiple time points across the day and night. A study of time budget measures in 174 horses living in eight facilities found that three sessions of 10 minutes, provided observations are made at different time periods when animals are calm, could give a good account of the local atmosphere. This finding supports the use of short, repeated observation sessions instead of continuous monitoring.

Step 2: Record Posture and Behavior

For each observation, record whether each animal is standing, lying in sternal recumbency, or lying in lateral recumbency. Also record whether the animal appears asleep or awake. In horses, standing sleep is characterized by a relaxed posture with the head lowered and eyes partially closed. Lateral recumbency is the typical REM sleep position in ungulates.

Step 3: Track Lying Time

The time spent lying is a meaningful welfare indicator. In the horse welfare study, the time spent feeding, in exploratory walking, and observation behavior were correlated with indicators of positive welfare and good practices, in contrast to the time spent standing immobile resting or in fixed attention and negative social interactions. Reduced lying time may indicate discomfort or stress.

Step 4: Monitor Changes Over Time

A single observation provides limited information. Track lying behavior over weeks and months to identify trends. A horse that previously lay down regularly but now stands continuously may be developing a health problem. The First Night Effect, where sleep duration and quality are compromised in unfamiliar environments, has been detected in horses using electroencephalography. Horses transported to new locations for competitions or breeding may show disrupted sleep patterns that resolve after acclimation.

Records and Measurements

Maintaining accurate records of sleep behavior supports early detection of problems. The following measurements are useful.

Lying Frequency

Count the number of times each animal lies down and stands up over a 24-hour period. The study of ungulate nocturnal behavior examined the time between two events of lying down in detail and found a high degree of rhythmicity in all species. Disruption of this rhythm may indicate a problem.

Lying Duration

Record how long each lying episode lasts. Short, frequent lying episodes may indicate discomfort, while long episodes of lateral recumbency suggest REM sleep. In horses, REM sleep duration averaged 46.1 minutes in one study, but ten horses exhibited REM-like sleep for less than 30 minutes while six exhibited REM-like sleep for at least 30 minutes.

Standing Sleep Episodes

Record when animals appear to sleep while standing. In horses, standing sleep is normal and expected. However, an animal that only sleeps standing and never lies down is likely REM deprived.

Environmental Conditions

Record temperature, lighting, bedding type, and noise levels. The study of lighting in stabled riding school horses found that the lighting period influenced total sleep, recumbency, wakefulness, and standing sleep. Strong artificial lights at night may affect sleep behavior and warrant further investigation.

Common Failure Patterns in Sleep Management

Several recurring problems can interfere with adequate sleep in farm animals.

Inadequate Bedding

Animals that lack soft, dry bedding may avoid lying down because of discomfort. This is a common cause of reduced recumbency in horses and cattle. The horse welfare study noted that time lying down was reduced in some facilities, and the effect of strong artificial lights at night needed further investigation.

Overcrowding

Animals that lack sufficient space to lie down safely may stand for extended periods. Dominant animals may prevent subordinate animals from accessing preferred lying areas. Observing social interactions during rest periods can identify this problem.

Pain and Lameness

Animals in pain often stand instead of lie down because rising and lying are difficult or painful. Equine gastric ulcer syndrome is a source of pain in affected horses and is associated with problem behaviors, some of which resolve following gastroprotectant therapy. However, problem behaviors persist in some cases despite gastroscopic resolution of disease, and some horses have pain-related learned anticipatory behavior even after the original source of pain has resolved.

Environmental Disturbance

Frequent nighttime disturbances by humans for management purposes disrupt sleep. The lighting study noted that modern horse husbandry involves significant time spent indoors, often in suboptimal lighting conditions and with frequent night-time disturbances by humans. Minimizing unnecessary nighttime activity supports normal sleep.

Transport and Novel Environments

The First Night Effect compromises sleep in unfamiliar environments. Horses transported to sporting events or breeding facilities may show reduced sleep quality. Electroencephalography data outperformed behavioral data in detecting the First Night Effect in horses, indicating the usefulness of EEG measurements for monitoring welfare or assessing stress and pain.

Welfare and Safety Context

Sleep is a fundamental biological need, and inadequate sleep has measurable consequences. The association between reduced REM sleep and impaired learning in horses demonstrates that sleep deprivation affects cognitive function. For working animals, this can translate into poorer performance and increased risk of accidents.

The welfare implications extend beyond performance. Chronic sleep deprivation is a stressor that can compromise immune function and overall health. The equine-assisted service horse study found that individual alterations in immune cell numbers and low cortisol could be associated with chronic stress. Four out of ten horses had high ACTH at baseline and had their medication readjusted, highlighting the importance of periodic blood tests.

For animal caretakers, the safety context includes both animal and human safety. An animal that is sleep deprived may be more reactive or less responsive to handling. Conversely, approaching a lying animal can startle it and cause a dangerous reaction. Understanding normal sleep postures and behaviors helps caretakers avoid unnecessary disturbances while ensuring that animals receive adequate rest.

Limitations of Standing Sleep Research

Research on standing sleep has several limitations that affect interpretation. Most studies of ungulate behavior are conducted during daylight hours, but nocturnal behavior patterns differ from those shown during day. The study of 18 ungulate species noted that detailed analyses of nocturnal behavior have only been conducted for very prominent ungulates such as giraffes, African elephants, or livestock, and the nocturnal rhythms exhibited by many ungulates remain unknown.

Behavioral observation cannot definitively distinguish sleep stages without electrophysiological confirmation. A preliminary study assessing sleep state in calves through electrophysiological and behavioural recordings addressed this challenge, but such methods are not practical for routine farm use. Behavioral criteria remain the primary tool for farmers, with the understanding that they provide an approximation of sleep state.

The distinction between rest and sleep is also important. An animal standing quietly with eyes closed may be resting instead of sleeping. The horse welfare study distinguished between standing immobile resting and other states, finding that time spent standing immobile resting was associated with negative welfare indicators. This finding suggests that not all standing rest is beneficial.

Professional Escalation Criteria

Farmers should seek professional advice when sleep behavior indicates a potential health problem. The following situations warrant veterinary consultation.

Persistent Absence of Recumbency

An animal that does not lie down for extended periods, typically beyond 24 to 48 hours, may have a medical problem. In horses, this can indicate colic, laminitis, or musculoskeletal pain. In cattle, it can indicate lameness or metabolic disease.

Sudden Change in Sleep Behavior

A marked increase or decrease in lying time compared to the individual's baseline warrants investigation. The rhythmicity of lying behavior in ungulates means that deviations from established patterns are meaningful.

Signs of Sleep Deprivation

Animals that appear excessively drowsy during the day, stumble, or show impaired coordination may be sleep deprived. In horses, short REM-like sleep duration without clinical signs of sleep disturbances was associated with poorer performance and lower perseverance in a learning test, indicating that even subclinical sleep restriction has consequences.

Concurrent Signs of Illness

Sleep changes accompanied by reduced appetite, weight loss, fever, or other signs of illness require prompt veterinary attention. The equine gastric ulcer syndrome commentary noted that horses with EGUS are often presented to veterinarians with a history of problem behaviors, some of which resolve following gastroprotectant therapy.

Behavioral Problems That Persist After Treatment

When problem behaviors persist despite resolution of the underlying medical condition, a behavioral medicine approach may be needed. This includes management of any underlying diseases, environmental modification, behavior modification, and in select cases, behavior-modifying medication.

Frequently Asked Questions

Why can horses sleep standing up?

Horses have a stay apparatus, a system of tendons and ligaments that locks the major joints of the legs so the horse can stand with minimal muscular effort. This allows light sleep while standing. However, the stay apparatus does not permit REM sleep, which requires muscle relaxation and therefore requires the horse to lie down.

Do elephants sleep standing up?

African elephants can sleep standing, and they do so for light sleep. However, they require recumbency for REM sleep. A study of nocturnal behavior in African elephants found that lying behavior increased during the night, consistent with the pattern observed across ungulate species.

Which birds sleep standing up?

Many bird species can sleep standing because of a tensegrity system in their legs. Rigid bones are held together by tension in tendons, allowing the system to stabilize under the action of gravity. This passive stability means birds can rest without muscular effort.

Is standing sleep as restorative as lying sleep?

Standing sleep is generally light sleep and is not as restorative as REM sleep, which requires lying down in most large mammals. REM sleep is associated with learning and memory consolidation. A study of horses found that longer REM sleep was associated with better performance in a spatial learning test.

How much REM sleep do horses need?

REM sleep duration varies among individuals. In one study of 16 horses, mean REM sleep duration was 46.1 minutes, but ten horses exhibited REM-like sleep for less than 30 minutes while six exhibited REM-like sleep for at least 30 minutes. Horses with shorter REM sleep showed poorer performance in a learning test.

Can walruses sleep in water?

Walruses can sleep while floating at the surface, lying on the bottom, or standing and leaning against the pool wall. REM sleep in water occurred in all positions. While in water, apneas lasted over 4 minutes, and all REM sleep episodes occurred during a single apnea.

Why do hippopotamuses sleep in water?

Hippopotamuses spend most of their nighttime hours in water. A study found that at night they spent on average 76% of their time in water. They can rest lying, sitting, or standing on the bottom with nostrils, eyes, and ears above the surface, or submerged and lying on the bottom.

How can I tell if my horse is getting enough sleep?

Observe your horse at multiple time points across the day and night. Record whether the horse is standing, lying in sternal recumbency, or lying in lateral recumbency. A horse that never lies down may be REM deprived. If you observe a persistent absence of recumbency or a sudden change in sleep behavior, consult a veterinarian.

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References and Further Reading

This article is educational and does not replace institutional policy, professional advice, or applicable safety and regulatory requirements.