Physiology of Sleep

Sleep is still incompletely understood, despite its clear importance in the lives of humans, dogs, cats, and all other mammals. It is important for normal brain function and cellular repair throughout the body. Wakefulness is promoted by the reticular activating system, also known as the ascending arousal system.¹ Sleep is initiated by inhibition of the arousal system. A number of neurotransmitters are involved, including catecholamines, histamine, acetylcholine, adenosine, and orexins.¹ Sleep pathways can be initiated by either hormonal signals such as melatonin, based on the circadian rhythm, or a homeostatic drive for sleep.² Sleep is inhibited by sympathetic nervous system activity and cortisol.²

Sleep can be broadly split into rapid eye movement (REM) and non-rapid eye movement (NREM) phases.¹ These have different functions, with cellular repair and immune regulation mainly occurring during NREM sleep, and neurological functions such as memory formation mainly occurring during REM sleep.²

Dogs and cats differ substantially in their normal sleep patterns, whilst the sleep pattern of dogs is similar to humans. Dogs sleep for approximately 10 hours each day, with the majority occurring at night.^3,4 They have approximately 20-minute cycles, with 12 minutes of NREM and 8 minutes of REM.³ Cats sleep for approximately 12 hours each day with a more flexible, less diurnal rhythm.^4,5 Thus, interventions to improve sleep in hospitalised small animals may have to differ depending on the species.

Sleep Deprivation in Critical Illness

Sleep deprivation, both in terms of quality and quantity, is a common problem in humans hospitalised in the ICU.⁶ There is currently no published research on the prevalence or severity of sleep disturbances in hospitalised small animals, but anecdotal experience suggests they are common and can be severe. Unpublished pilot research from our institution suggests that dogs hospitalised in the ICU are frequently awake at times in the night that they should be asleep.

It is intuitive to state that sleep deprivation is an animal welfare concern. However, there is a growing body of evidence to support that it has substantial and wide-ranging adverse effects on many body systems. In humans, sleep deprivation has been associated with impaired immune function, alteration of neurohormonal axes such as the thyroid and adrenal axes, and alterations in pulmonary mechanics.⁷ Sleep deprivation can also contribute to hospital-associated anxiety,⁸ a substantial problem that is further discussed in the proceeding for Anxiety in Hospitalised Patients: Recognition and Management.

Strategies to Promote Sleep

There is minimal published research investigating methods to promote sleep in hospitalised small animals. This is also an area where research from critically ill humans is often difficult to extrapolate to small animals. Thus, most of the recommendations in this section carry a low level of evidence to support them. However, they mostly make physiologic and practical sense. Further research is urgently needed.

The first step is to recognise when the problem is present. Patients should be monitored for whether they are sleeping. Tracking sleep patterns on patient charts may identify a sleep problem with a consistent pattern. Assessing whether an animal is undergoing good quality sleep is challenging, and further research is required in this area. To improve sleep, broad strategies include promoting an ideal environment for sleep, treating concurrent conditions that inhibit sleep, and pharmacological adjuncts.

Promoting an Ideal Environment for Sleep

Intensive care units are busy places, where noise and light levels are often not conducive to sleep. A study of two veterinary ICUs showed both high average noise levels and frequent sound spikes,⁹ both of which have been shown to contribute to sleep disruption in hospitalised critically ill humans.^10,11 Our pilot study had similar findings. Bright lighting can also be disruptive to sleep, especially when it disrupts the normal circadian rhythm.¹¹ However, dogs in a shelter setting can sleep well in the light, suggesting this is not the most aversive part of the ICU environment.¹² Light conditions were also not significantly associated with wakefulness in our pilot research, though this was likely underpowered. In human ICU patients, sleep quality can be improved by the use of earplugs and eye masks.¹³ This is not likely to be directly translatable to small animals, which are not expected to tolerate such interventions. Thus, approaches should centre on the control of environmental sound and light levels. Protocols can be developed for enforced quiet lights-out time.¹⁰ As caregiver interactions may also disturb sleep, only strictly necessary patient treatments should be scheduled during this time.¹¹ ICU design can incorporate quiet areas and the ability to selectively illuminate specific portions of the ICU to avoid excessive artificial light. Sound absorbing building materials can reduce reverberation time, which is how long sound persists once the source has ceased.¹⁰ Sound can be masked by white noise or music.¹⁰ Further research is needed into the effectiveness of these strategies in veterinary ICUs.

Treating Concurrent Conditions that Inhibit Sleep

Whilst sleep deprivation can contribute to anxiety, untreated anxiety can also contribute to an inability to sleep. Treatment of anxiety is discussed in the proceeding for Anxiety in Hospitalised Patients: Recognition and Management. Other unpleasant sensations, such as discomfort, pain, and dyspnoea, may also inhibit sleep and should be treated appropriately.¹¹ It is also prudent to note that some medications will have adverse effects on sleep, such as opioids, methylxanthines, and sympathomimetics.^11,14 Attention should be paid to current medications in a patient that is unable to initiate or sustain sleep. Separation from familiar conspecifics in pack dogs may lead to impairment of sleep quantity and quality.¹⁵ Familiar smells may therefore aid sleep by mimicking proximity of familiar individuals.

Pharmacological Adjusts

Sedatives may be useful in promoting sleep. However, it should be noted that not all sedatives promote natural sleep. For example, benzodiazepines may actually impair good-quality sleep by causing an increase in circulating catecholamines.¹¹ a₂ agonists such as medetomidine or dexmedetomidine provide sedation that more closely mimics natural sleep than other sedatives.¹⁶ This may be due to convergence on sleep pathways at the locus coeruleus, part of the ascending arousal system. Medetomidine boluses of 0.5–2.0 mg/kg can be used to assist in initiation of sleep. Timing of bolus administration to coincide with quiet time and minimal handling is likely to increase benefit. Administration of a CRI at 0.5–2.0 mg/kg/hour may be beneficial for some patients. Doses should be halved if using dexmedetomidine rather than medetomidine, but dexmedetomidine may provide less sedation.¹⁷

Given its role in promoting onset of sleep, oral supplementation of melatonin is an attractive strategy for improving sleep in the ICU. In one study of critically ill humans, melatonin supplementation was more effective at promoting sleep than earplugs and eye masks, and led to a significant increase in serum melatonin concentration.¹⁸ Melatonin has not been investigated as a sleep aid in hospitalised small animals. However, its use for other problems has provided some preliminary evidence for safety in dogs and cats.¹⁴ Suggested doses include 0.1 mg/kg for dogs and 1.5–6.0 mg (total dose) for cats, 30 minutes prior to the desired onset of sleep.¹⁴

Conclusions

There is an urgent need for further research into the sleep patterns of hospitalised small animals, the impacts of sleep disruption, and strategies for promoting sleep. In the meantime, implementation of simple strategies such as protocols to reduce noise and light levels is inexpensive and likely to be helpful. Medical therapies such as medetomidine and melatonin could be considered in some cases, however there is currently minimal evidence to support their efficacy and safety.

References

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