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Inactive BehaviorMouse Ethogram

Group Sleeping

By November 15, 2019No Comments


Group sleeping, as the name implies, is a type of sleeping behavior where a group of mice collectively sleeps together. Since mice are a social species, group sleeping comes naturally to them.


Group sleeping is an inactive behavior. Because mice follow a sleep-wake cycle, their active behaviors are followed by a long period of inactivity. Mice are naturally nocturnal (they sleep during the day) and, as with many animals at risk of predation, they prefer to engage in group sleeping, i.e. sleeping at the same time and in the same place as other members of their nestmates.

Since group sleeping is done in numbers, it can be considered as an affiliative behavior. During group sleeping, many mice are sleeping together and are bundled up or in close proximity to one another. This generally occurs in a designated sleeping area. Sometimes a mouse may participate in individual sleeping, but this is typically when the rest of the colony is active.

When housed in a cage or other small enclosure, mice will naturally apportion different areas of the living space for different purposes, and one of these socially agreed divisions will become a group sleeping area. The sleeping area is usually opposite the area designated for the disposal of feces and urine; this coordinated action allows the mice to collectively avoid the diseases that could arise from prolonged contact with waste material, as well as to reliably engage in the group sleeping behavior.

Details of Behavior

When mice sleep in a group, they usually adopt one of two (or a combination of these two) well-known conformations:

  • the dogpile: mice sleep on top of each other, with some mice having parts of their body (or even their entire body) covered by other mice
  • huddling: mice sleep alongside one-another, with each mouse having the whole of one or both sides of its body pressed against the side of another mouse.

Rarely will a group of sleeping mice exhibit purely one conformation at a time.

The Challenges of Studying Group Sleeping in Mice

Conducting behavioral research is challenging, especially so when it comes to housing mice in groups. The major challenge of studying group sleeping in mice is male mice’s tendency towards aggressive behavior. So, even though mice are a social species, male mice are very likely to exhibit aggression when housed together, making it difficult to study group sleeping in male mice than female mice.

A study conducted by Van Loo et al. in 2003, set out to determine the complexity of the inter-male social contact in a series of tests that examined whether males prefer to be in an empty cage or near a familiar cage mate. The experiment showed that male mice prefer to spend more time in the proximity of a familiar cage mate than by the empty region of the cage. In fact, male mice display preference in sleeping closer to their familiar cage mate. Mice did not have separate sleeping areas, meaning they shared a sleeping area, and they were usually found sleeping in close body contact with each other

Related Behaviors

When studying group sleeping behaviors in mice, consider gaining a deeper understanding of these related behaviors as well:

  • Sleeping. Sleeping is a complex behavior in it and of itself which takes up a significant portion of a mouse’s lifetime. Sleep is described based on the presence of rapid eye movements (REM) or non-REM as revealed through measurements acquired from electroencephalograms and electromyograms. Without having adequate sleep, mice become unhealthy. Therefore, sleeping is also classified as a maintenance behavior which is crucial for maintaining homeostasis.
  • Huddling. Huddling refers to the physical contact that occurs between mice when they are lying down clustered together.

Function/Physiology of Behavior

It is believed that mice practice group sleeping for the following reasons:

  • For collective security. The adage “safety in numbers” captures the essence of group sleeping. Being in a group reduces the risk of being the individual chosen for pursuit by the predator because it is difficult for a predator to single out a mouse from an entire group. The predator may also be wary of increased intensity of defensive strategy when facing a large group. Finally, being in a group increases the probability that at least one member will become aware of an impending ambush before it happens, and so be able to “warn” the others to wake up and flee the threatening situation.
  • For better temperature regulation. Although mice are endothermic animals (able to regulate their body temperature through internal metabolic and mechanical processes), they still perform behaviors that aim to enhance their temperature regulation. A large group of mice sleeping together in an enclosed space will generate a lot of heat, and so every individual will be able to benefit from the heat. This is important for survival as it increases the group’s chances of enduring harsh temperatures.
  • To reinforce social bonds. Since group sleeping is a type of affiliative behavior in mice, it can reinforce the colony’s social bonds due to the mutual benefit that goes to all the mice as a result of this social interaction.
  • To reduce the risk of diseases. Since mice group sleep in areas that are strategically chosen, such as away from where they urinate, this ultimately reduces their collective risk of disease. By sleeping in clean areas, the whole nest of mice increases their chances of survival.

Application of the Behavior

Under the following circumstances, mice are likely to exhibit sleeping behavior:

  • Under group-housing conditions. 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. When a mouse doesn’t sleep after the expected active period has passed, it’s considered to be an abnormality.
  • During the day. Since mice are nocturnal creatures, they are most likely to sleep during the day and be active at night. Mice that are active during the day are considered to be abnormal.

Research Techniques for Studying Group Sleeping

The research techniques that are applied to study sleeping behaviors in individual mice can be applied to multiple mice in order to study group sleeping:

  • Behavioral studies. Behavioral studies make use of behavioral tests for assessing how sleep, cognition, and behavior are related in mice.
  • 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 the stages of sleep.

Behavioral Tests for Measuring Group Sleeping in Mice

To observe group sleeping, researchers use video to record mice across their sleep-wake cycle in order to see how often the mice sleep together.

It is also possible to categorize group sleeping in mice by scoring the presence of group sleeping as 1 and individual sleeping as 0.

Mouse Strains

C57BL/6J Mice

C57BL/6J mice are typically used as comparator mice in behavioral research. These mice demonstrate high levels of group sleeping behaviors, even when there are mostly males present in group housing conditions.

3xTg-AD Mice

3xTg-AD mice serve as a mouse model of Alzheimer’s disease. It is the only model that exhibits both tau and Abeta pathology which are also found in the human form of Alzheimer’s disease. While these mice show many deficits, such as increased anxiety and startle responses, they demonstrate normal levels of group sleeping behavior, comparable to that found in C57BL/6J mice.


Striatal-Enriched protein tyrosine Phosphatase (STEP) is a protein that is neural-specific and opposes synaptic strengthening. STEP is associated with brain regions that have otherwise been implicated in social behavior, including the hippocampus’ CA2 region, the cortex, amygdala, and the striatum. In psychiatric and neurodegenerative disorders, abnormal levels of STEP have been found. STEP KO mice which have been genetically programmed to lack this gene have their social, affiliative behaviors impaired. For example, they demonstrate impaired social memory in the Three-Chamber Test. However, the STEP KO mice demonstrate normal levels of group sleeping behavior, similar to C57BL/6J mice, indicating that group sleeping as a social behavior remains unaffected by this genetic manipulation.


Mixed results have been found regarding BTBR mice, a mouse strain commonly used to model autism spectrum disorder. While some studies demonstrate significantly lower instances of huddling and group sleeping, others have found that BTBR mice group sleep as frequently as C57BL/6J mice do.

BALB/cByJ Mice

BALB/cByJ mice, which are a substrain of BALB/c mice, demonstrate group sleeping behaviors in the same way that C57BL/6J mice do. They demonstrate high levels of group sleeping and even in male-dominated mouse colonies, high instances of group sleeping will still be observed.

BALB/cJ Mice

BALB/cJ mice, in contrast to BALB/cByJ mice, sleep alone more than C57BL/6J mice do. This means that BALB/cJ do not display as much group sleeping as these comparator mice. However, in some studies, BALB/cJ mice are not statistically different from C57BL/6J mice in their group sleeping tendency. So, a consensus has not yet been firmly established.

NIH Mice

NIH mice are very likely to be seen resting and sleeping alone. They demonstrate more isolated behaviors than either C57BL/6J mice or BALB/cJ mice do. Thus, NIH mice demonstrate less group sleeping than BALB/cJ mice which already show lower levels of group sleeping than C57BL/6J mice do.

129S1/svImJ Mice

129S1/svImJ mice are a substrain that is commonly used as a background strain for genetic modeling for behavioral research, just as C57 mice are used. 129S1/svImJ mice, however, are characterized as a hypoactive anxious behavioral phenotype. While these mice are anxious and display altered levels of cephalocaudal grooming, they also demonstrate lower instances of group sleeping behavior. While C57BL/6J mice group sleep 100% of the time, as reported by some studies, 129S1/svImJ mice group sleep together about 80% of the time.

Dvl1 Mice

Dvl1 mice have a null allele for the Dishevelled gene, which code for Dishevelled proteins. The Dvl1 gene, along with Dvl2 and Dvl3, have been found to be expressed broadly in healthy adult tissues, and also during embryonic development. Dvl1 mice (which lack the Dishevelled gene)  demonstrate abnormal social behavior and deficits in sensorimotor gating as measured by the Acoustic Startle Response. In terms of their social behavior, Dvl1 mice have lower than normal instances of group sleeping and huddling than wild-type mice do. This home cage behavior abnormality extends to include deficits in nest-building behavior, as well.


Individual Sleeping

Since mice are considered to be a social species, individual sleeping can be seen as an abnormality.

In some rare cases, individual mice will choose to sleep alone or to defect from group sleeping to engage in an active behavior such as exploring the environment.

A 2004 study examining the sleeping habits of sixty mice from the BALB/c strain reported very rare defection from group sleeping behavior and none of this defection in juvenile mice. Even amongst mice with a very high basal level of aggression, such as the Swiss strain, group sleeping and nesting has still been observed.

Data on abnormalities of mice group sleeping behavior is scarce in peer-reviewed literature.


  • Group sleeping is a type of sleeping behavior where a group of mice collectively sleeps together.
  • Group sleeping is an affiliative, inactive behavior.
  • When mice sleep in a group, they will do so either in a huddle or dogpile formation.
  • The major challenge of studying group sleeping in mice has to do with male mice’s tendency towards aggressive behavior when housed together.
  • Mice group sleep for several reasons, including: for collective security, for better temperature regulation, to reinforce social bonds, and to reduce the risk of diseases.
  • Group sleeping can be observed under group-housing conditions, when a mouse is tired, or during the daytime.
  • Behavioral studies, as well as EEG and EMG recordings, are commonly used techniques for studying group sleeping in mice.
  • C57BL/6J mice demonstrate high levels of group sleeping behaviors, even when there are mostly males present in group housing conditions.
  • The following mice are considered to be abnormal but maintain healthy, normal group sleeping patterns: 3xTg-AD mice, STEP KO mice, BTBR mice, BALB/cByJ mice, In some studies BALB/cJ mice demonstrate low group sleeping levels while in other studies they do not.
  • The following mice display significantly lower levels of group sleeping than normal C57BL/6J mice: NIH mice, 129S1/svlmJ mice, and Dvl 1 mice.
  • Individual sleeping is an abnormality of group sleeping.


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