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The Deep Sleep

By Laura Hooper

Photo credit: Wikimedia Commons

It is getting cold and dark outside as winter draws in. Doesn’t it just make you feel sluggish and tired? Like you want to just snuggle in bed? This is exactly what some members of the animal kingdom do, by hibernating to avoid the harshness of winter.

Hibernation is the act of falling in to a state of inactivity (deep sleep), which results in a major slowing down of bodily functions and suppression of metabolism [1]. The reason animals hibernate is to conserve energy, as during the winter there is a large reduction in their food sources [2]. In addition, by hibernating they don’t have to keep regulating their temperature to keep warm, which is another way to conserve energy. There are many animals that hibernate such as; primates like the fat tailed dwarf lemur, rodents, butterflies, bears and even some birds such as the common poorwill [1].

The length of hibernating periods depends on the animal and may last days, weeks or months [1]. There are two different types of hibernator [2]. Firstly there are the true hibernators; these enter such a deep sleep that they appear dead! Their body temperature drops and their heart beat slows. Bears are the largest true hibernator and they can sleep for up to 6 months. Other true hibernators include the little brown bat and woodchucks [2]. Then there are other hibernators that only enter a light sleep to avoid the most severe weather periods. There are no major changes in their temperature, heart rate or breathing. Examples of these include; raccoons and skunks [2].

There are a number of things that animals do to prepare for hibernation. They may store food in their burrows, which can be eaten if they wake up for short periods of time [2]. They will also ‘fatten up’, by consuming more food in the autumn when it is plentiful. In fact body mass can be increased by as much as 50% in the months leading up to hibernation [3]. In addition, more fat is laid down, both white and brown types, this will be discussed fully later. [2]. All this is done so the animals have a large enough energy store for the entire period of hibernation.

This preparation occurs over many months and the cues to begin this are thought to be photoperiodic, for example day shortening [2]. They sense the changes with the help of their circadian clock. So how does this clock work? It all depends on clock genes within cells switching between increased and reduced expression:

1. Clock genes are stimulated and have enhanced expression

2. This results in particular proteins being made

3. These proteins are negative factors and inhibit expression of the clock genes

4. This is a case of a gene product supressing its own expression

The cycling between these two states of gene expression is like a pendulum swinging in a clock, resulting in accurate timekeeping [4].

Photo credit: L.Hooper

Once the animal senses the days getting shorter, temperature changes or food shortages it triggers the release of the ‘Hibernation inducement trigger’ [2]. This is an opiate that is related to morphine which when released into the blood triggers hibernation. There is still mystery surrounding how and why this occurs [2].

A number of significant physiological changes happen during hibernation. Temperature and heart rate are greatly reduced for example a Woodchuck’s heart rate slows from 80 to 4 beats per minute and temperature falls from 36.6°C to 3.3°C [2]. The major change is the reduction in metabolic rate which can be suppressed to as little as 1-5% of the normal level [3]. All these reductions are done to save energy. In addition, metabolism usually primarily involves carbohydrate, e.g. glucose, breakdown but during hibernation this is switched to fatty acid metabolism [3]. This switch is achieved by activating hibernation specific genes that produce products and molecules that are required to do this. The fatty acids are obtained from what was stored as white fat during the preparation period [4]. They switch otherwise he animal would run out of carbohydrates to metabolise during hibernation. Although they aren’t consuming food, they have previously stored fats which can be slowly broken down (catabolism), to provide energy to create new products (anabolism) [3]. Both the breaking down and generation of new products from this are part of metabolism.

The role of brown adipose

White fat is a lipid store and is used as a source for metabolism [4]. The other type of fat is brown fat (adipose), this is not used for fat storage but is instead accumulated before hibernation to provide heat [4]. Heat production is required for arousal which is the “process of rewarming the body by metabolic heat production” [4]. The reasons for arousal include [3]; to eat briefly, eliminate waste, readjust neural circuits and recharge the immune system. But these are highly debated and differ between species.

Brown adipose is crucial to this process and is adapted in its structure in several ways for this role such as having a rich blood supply and large numbers of mitochondrial cells [4]. Mitochondria are usually responsible for ATP (Adenosine tri-phosphate, the molecule that carries energy within the body), production which is a stage in respiration. However, in brown adipose tissue the mitochondria can be uncoupled from this process by uncoupling protein 1. This protein is found on the inner membrane of the mitochondria and it prevents ATP production, instead causing the mitochondria to generate large amounts of heat. This process is triggered when a chemical called norepinephrine is released into the fat tissue when arousal is needed [4]. This fat is found in patches around vital organs e.g. heart, brain and lungs, this allows quick arousal once heat is produced [2]. Once the adipose has heated the area to around 15°C shivering of the skeletal muscles will take over to rewarm the whole body [3].

Photo credit: Wikimedia Commons

This approach is a survival mechanism that has evolved to conserve energy giving the animals a fighting chance at surviving the harsh elements during winter to emerge in spring. So when you feel the cold this winter and crank up the heating or put on your best Christmas jumper spare a thought for the animals out there who are sleeping to survive.


  1. ’Hibernation’, Wikipedia: (Accessed 3/11/15)
  2. ’Hibernation-How animals survive the winter’, Science made simple: (Accessed 3/11/15)
  3. P.Morin Jr. and K B. Storey (2009), ‘Mammalian hibernation: differential gene expression and novel application of epigenetic controls’, INT. J. Dev. Biol. 53. Pg. 433-442
  4. R W. Hill et al. (2008), ‘Animal Physiology’ 2nd edition, Sinauer Associates Inc.


  1. Wikimedia commons, Black grizzly bear: (Accessed 12/11/15)
  2. Circadian clock process figure created using information from page 388 in reference 4.
  3. Wikimedia commons, Mitochondria: (Accessed 4/11/15)

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