Sleeping is a motionless behavior where in a mouse can be found resting and not awake. Occasionally, the mouse may involuntarily have a muscle twitch, but this is not a goal-directed movement. A mouse can sleep in isolation or in the presence of other familiar mice.
Sleeping is an inactive behavior characterized by the lack of locomotion.
Without having adequate sleep, mice become unhealthy. Therefore, sleeping is also classified as a maintenance behavior which is crucial for maintaining homeostasis.
While sleeping, a mouse is resting. Its body is curled up and its face is typically tucked in. Sometimes, a mouse may be sleeping while curled up in a sitting position.
REM vs. Non-REM Sleep
Usually, when studying sleep, behavioral scientists are interested in describing the brain oscillatory patterns which occur throughout the observation period. Therefore, sleeping behavior is classified based on the presence of rapid eye movements (REM) or non-REM as revealed through measurements acquired from electroencephalograms and electromyograms.
Non-REM is characterized by delta activity brain waves which are large and slow in nature and fall below 4 Hz when measured by EEG. REM sleep, on the other hand, is characterized by rapid brain oscillations which resemble those observed during wakefulness. REM is reflected by EEG recordings which are in the theta range, falling between 6 to 10 Hz.
Group Sleeping: A Type of Sleeping Behavior
Group sleeping is an affiliative behavior of interest to many researchers. During group sleeping, many mice are sleeping together and are bundled up or in close proximity to one another. Group sleeping is a social phenomenon and you can read more about it here.
Function/Physiology of Behavior
Sleeping behavior have the following functions in mice:
- To avoid predators. Mice sleep during the day and are nocturnal creatures. By sleeping during the day and staying awake at night, mice avoid being attacked at night (thus, can stay awake and avoid nocturnal predators such as owls and racoons).
- To regain energy. Adequate sleeping levels are associated with health. Therefore, sleep is crucial for maintaining homeostasis and surviving optimally. In fact, during sleep, genes which are involved in metabolism regulation are more enhanced than in wakefulness.
- To consolidate memory. Memory is consolidated or made stronger during sleep. Particularly, theta rhythms are said to play an important role in this process. Diseases which are characterized by disturbed sleep and cognitive problems, such as type 2 diabetes and Alzheimer’s disease, also have theta rhythm abnormalities.
- To clear brain metabolites. Accumulated waste and reactive oxygen species may lead to damaged cells and tissues. However, while sleeping, there is a higher clearance of waste brain metabolites than when awake. Therefore, sleep may serve the function of clearing the metabolic waste which normally accumulates during waking hours. So, sleep plays a beneficial role that aids in survival and health.
- To foster collective security. When mice are sleeping together, as is the case in group sleeping, a sense of security is fostered. An individual mouse is less likely to be attacked by a predator when in the midst of so many other mice. Also, if a single mouse senses a predator and wakes up it may alert the others, thus providing an additional layer of security which can aid in survival.
- To regulate body temperature. In instances of group sleeping, where many mice are huddled together or in close proximity to one another, body temperature is regulated in the sense that the emanating body heat will keep the mice warm. Staying warm collectively is an important function for ensuring survival through cold temperatures.
Application of the Behavior
Under the following circumstances, mice are likely to exhibit sleeping behavior:
- Under group housing conditions. In the case of group sleeping, mice are likely to exhibit this behavior under social living conditions where the cage is shared with many other mice. Mice are very likely to display group sleeping in such housing conditions since they are social creatures. Even mice with a naturally high level of aggression, like the Swiss strain, are apt to group sleep.
- When tired. Mice are likely to sleep after several hours of activity, in order to restore their energy levels.
- During the day. Since mice are nocturnal creatures, they are most likely to sleep during the day and be active at night.
Research Techniques for Studying Sleeping
- Pharmaceutical studies. Sleeping is a popular topic explored by pharmaceutical studies using animal models. Since lack of sleep is a contemporary problem affecting health and well-being, a lot of resources are being placed into behavioral studies in order to find possible solutions to sleep-related problems such as insomnia.
- Behavioral studies. Behavioral studies make use of behavioral tests for assessing how sleep,cognition, and behavior are related in mice. For example, if the research question is whether sleep deprivation can increase anxiety levels in mice, appropriate behavioral tests, such as the Forced Swim Test (which is a validated test for assessing anxiety levels in mice), will be used when conducting the study.
- Circadian gene expression studies. In order to determine how circadian genes vary across conditions, control and experimental mice will be sacrificed and their brain tissue (specifically, the hypothalamus) will be analyzed.
- Optogenetic stimulation. Optogenetic stimulation can be used to target specific genes and stimulate them through light activation, thereby studying the effect of genes on behavior actively. Current sleep research trends are investigating how mice’s sleep is affected by astrocyte activation in the hypothalamus.
- Electroencephalogram and/or electromyogram recordings. Mice studies enable researchers to acquire electroencephalogram (EEG) records by placing electrodes through drilled holes in the skull, in order to place the electrodes directly on the brain and reduce noise. The same applies to electromyographic (EMG) records, electrodes are subcutaneously inserted on the nuchal muscles. Such techniques are used to collect data on the mouse’s physiology and on the stages of sleep.
Since sleeping is an inactive behavior, there are not many behavioral tests available for measuring it. However, the following two techniques are used when studying sleeping behavior in mice, in order to gather as much data around this passive behavior as possible:
- Wheel Running Assays. The wheel running can be used to measure circadian behavior in mice. Activity during the day is compared to activity during the night, in order to establish a nocturnal, circadian pattern. Therefore, bouts of inactive activity, such as sleeping, greatly influence the data and activity recorded by this assay.
- Video analysis. Video capturing can make it easier to identify and quantify moments of completely inactive behavior. This technique, although useful and efficient, is limited, in that it cannot distinguish between bouts of rest and actual sleep (something that EEG can do).
Pharmaceutical Studies on Sleeping
Gelsemine Alleviates Sleep Disturbance
It is estimated that over 70% of patients with a chronic pain condition are likely to complain of sleep-related issues. One study demonstrated, using partial sciatic nerve ligation mice which are known to model chronic pain, that gelsemine administration is able to improve sleep in these experimental mice by increasing the total number of episodes of non-REM sleep compared to positive controls.
Tenuifolin May Enhance Sleep in Mice
Tenuifolin is a saponin derived from the dried root of Polygala tenuifolia, which has been demonstrated to have anxiolytic and hypnotic properties. Mice which are given this extract will sleep longer and have longer bouts of both non-REM and REM sleep. Mice which are given this supplement are less active during the dark phase since they are sleeping while normal mice are usually energetic during the night.
Benzodiazepine May Create Cognitive Problems
Benzodiazepine is a commonly used drug for inducing sleep. One study demonstrated that learning and cognitive deficits were associated with mice which were treated with triazolam, a benzodiazepine, during their juvenile period as concluded by results showing poorer performance in the learning-dependent Fear Conditioning Task. Such a finding shows how drugs which aim to target sleep-related circuits in the brain may have harmful effects if studied in a different context.
Strains Exhibiting Sleep
Even though sleep is common to all mice, it may not be stable and exhibited in the same way across all strains as conditions vary. In fact, the following is an example of how adverse conditions affect sleep differently across mouse strains:
Varying Effects of Contextual Fear on Sleep Across Strains
Behavioral experiments studying sleep demonstrate how certain exposures can impact sleep outcome in mouse strains. One experiment demonstrated how Fear Conditioning altered the sleep response for commonly used lab mice in the following ways:
- BALB/cJ mice have reduced REM sleep after being subjected to the Fear Conditioning Task.
- Hybrid CB6F2/J mice have a large reduction in REM sleep due to foot shocks.
- C57BL/6J mice do not have their sleep affected by foot shocks as much as the other two strains and, thus, are more resilient.
Such findings demonstrate how fearful and aversive conditions impact the quality of mice’s sleep according to strain.
Abnormalities in Behavior
Mother’s Chronic Low-Protein Diet Affects Offsprings’ Sleep Patterns
Young pups whose mothers were fed a low-protein diet while pregnant will end up having an altered sleep pattern. Male mice whose mothers did not have adequate protein levels when pregnant will rest and sleep longer at night when compared to normal mice which are naturally more active during the night. More research should be conducted in order to establish how maternal diet affects sleep in female offspring.
Altered Sleep in Prokineticin 2 Deficient Mice
Prokineticin 2, a protein believed to be an important molecule of the hypothalamic suprachiasmatic nucleus, is involved in maintaining the rhythmicity of circadian patterns. Mice which are genetically manipulated to have the prokineticin 2 gene knocked out have altered sleeping patterns, including a decrease in non-REM sleep and a decreased amount of sleeping during the day/light period when mice are expected to be sleeping.
The Dp(16)1Yey/+ mouse model (Dp16) is a relatively new method of modeling Down syndrome (DS) in mice. It is considered to be a great development because it lacks genes which are unrelated to DS, an issue which was previously a major confounding variable and a cause for concern for scientists using those older models.
In this DS mouse model, sleep abnormalities have been noted when compared with wild-type mice by using data collected through implanted EEG electrodes to compare differences between the groups. The DS mice spend more time awake during both the day and night. However, DS mice spend less time in the non-REM brain state during the dark phase when compared to wild-type mice.
Possible explanations for these characteristics include: Dp16 mice require less sleep because they have longer bouts of non-REM sleep or they have more difficulties staying asleep and, therefore, are more likely to transition between phases of sleeping and wakefulness.
Sleep Abnormalities in Non-Metastatic Breast Cancer Model
Mice which have been used to model a non-metastatic form of breast cancer have changes in their sleep patterns when compared to wild-type mice. The reason for this is believed to be related to altered activity levels of hypocretin/orexin (HO) neurons of the lateral hypothalamus. One experiment shows that while tumors were developing in mice, their sleep profile was becoming significantly altered when compared to controls. The experimental mice had reduced time in the awake-phase and their sleep was also more fragmented, indicating that mice with tumors were unable to stay fully awake during the light phase
Encephalitis-Related Sleep Changes in Mice
Encephalitis antigens can be found in several regions of the brain which are involved in sleep. Mice induced with viral encephalitis via the intranasal application of vesicular stomatitis virus will show an alteration in their sleeping patterns, including a decrease in REM and an increase in non-REM sleep.
Disease Models of Sleep
Sleep Deprivation Models
Sleep deprivation using the Wheel Running Assay has already been firmly established in rats but has only recently been applied to mice. Using the mouse sleep deprivation model, mice are kept on a tight schedule in which they are subjected to the activity on the wheel. This model is useful because mice will have a 97% reduction in the amount of sleep they have during the light phase and will not have a significant increase of corticosterone in their blood, indicating this model is not stressful to the mice.
Insomnia is one of the most common sleep disorders in the human population. It is also incredibly difficult to model it accurately in mice. Although there are a few methods, which include sleep deprivation, administration of caffeine to delay sleep-onset, and creating brain lesions, these models are not exactly reflective of the human situation, since the mouse, unlike human patients, has both the ability and desire to fall asleep.
Narcolepsy in humans is defined by the presence of cataplexy, sleep paralysis, daytime sleepiness, and hypnagogic hallucinations. Cataplexy is the most prominent feature of narcolepsy wherein muscle atonia (and, thus, complete postural collapse) suddenly occurs during wakefulness. In humans, narcolepsy is usually associated with disturbed sleep, prolonged non-REM and REM cycles, OSA, and sleep fragmentation.
Narcolepsy in mice is studied using orexin-deficient mice given that human patients also have low levels of orexin in the cerebral spinal fluid, so there are some similarities between the mouse model and narcolepsy in humans.
In humans, sleep apnea occurs when there is a reduction or a complete stop of airflow while sleeping. Obstructive sleep apnea (OSA) is more common in humans than the other form, central sleep apnea. In humans, OSA is comorbid with serious conditions such as cardiovascular disease and diabetes.
In mice, OSA appears spontaneously after experiencing acute hypoxia. By exposing the mouse to acute hypoxia, researchers study relationships between OSA, age, obesity, and neuromuscular control. One of the major drawbacks of such a model is that it reflects an extreme form of OSA that may not be applicable to the majority of clinical instances of OSA.
Hypersomnia is a disorder of excessive sleepiness which affects awake and alert states, thus influencing awake but still behaviors. Hypersomnia is characterized by the difficulty or inability to stay awake. In mice modeling hypersomnia, the awake but still state will be less observed since mice are spending more time in the sleep-phase.
In order to induce hypersomnia, a mouse can be induced to have high levels of anxiety or depression which further affects a mouse’s sleep behaviors by increasing time spent in the sleep-phase. Thus, chronic anxiety and depression increase corticosterone levels which alter a mouse’s normal sleep schedule by decreasing the overall number of wake episodes. This, in turn, leads to a decrease in the overall number of displayed awake but still episodes.
- Sleeping is an inactive behavior characterized by the lack of locomotion.
- While sleeping, a mouse is resting. Its body is curled up and its face is typically tucked in. Sometimes, a mouse may be sleeping while curled up in a sitting position
- Group sleeping is a commonly observed variant of sleeping wherein many mice are sleeping together bundled up or in close proximity to one another.
- Sleep serves many functions in mice since it helps them to: regain energy, consolidate memory, avoid predators, clear brain metabolites, foster collective security, and regulate body temperature.
- Sleep studies in mice can use a variety of techniques and methods, including pharmaceutical interventions, behavioral studies, gene expression, optogenetic stimulation, and EEG and EMG recordings.
- Pharmaceutical studies demonstrate how certain drugs and supplements may impede sleep while others may promote it.
- Mouse strains will vary in their sleeping pattern based on their environmental conditions. For example, if a mouse is in a fearful environment, their sleeping pattern will change from what it was at baseline.
- Diseases and conditions, such as a mother’s diet, genetics, cancer, encephalitis, and Down’s syndrome, will have an impact on a mouse’s sleeping profile.
- Many models of sleep exist used for studying sleeping disorders in mice, including insomnia, hypersomnia, narcolepsy, and sleep apnea.
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