Hibernation is commonly described as a form of extended sleep, but it is not accurate because it reduces activity and depressed metabolism. However, it can vary from a deep unconsciousness to light spells of inactivity among different species. Certain animals, including some mammals, reptiles, amphibians, and insects, undergo hibernation to survive extremes of temperatures or stressful conditions. In addition, it functions to conserve energy when enough food is not available. Researchers are studying to promote longevity in humans by inducing a state of hibernation. It can be helpful for many reasons, for example, temporarily hibernating seriously ill or injured people until treatment can be given.
Hibernation is a state in which hibernating species undergo various changes such as changes in metabolism, drop in body temperature, and reduction in breathing and heart rate. Researchers are interested in knowing how hibernation works by avoiding health problems and returning to total activity after the hibernation period in animals.
Hibernation and Aging
Aging is a common factor determining lifespan. But the characteristics that determine the rate of aging are not still clearly known. Recent discoveries have shown that long-lived mammalian species have one thing in common: the ability to depress metabolic rate and undergo hibernation in times of environmental stress. Scientific research has shown that the ability to hibernate arose in mammalian ancestors around 65 million years ago, which is common to most mammals today. It means that the genes in those ancestors allowing them to hibernate are likely to be present in humans, but we are not using them. So, by knowing the genes working during hibernation, scientists might target the same genes in humans for inducing hibernation, thereby delaying aging.
Not only do animals hibernate, but historical evidence from fossils suggests that once humans also used to undergo hibernation during extreme winters. Evidence was studied from the bones found during excavations in Sima de Los Huesos, near Burgos in Northern Spain, suggesting that our hominid predecessors may have used a state of hibernation during harsh winters. The signs of damage and the lesions found on these fossil remains were the same as those in the bones of other animals that hibernate. These fossils were probably from Neanderthals dating back more than 400,000 years. A paper published in the journal L’Anthropologie suggested that the fossils found and studied showed seasonal variations that point towards the fact that bone growth was disrupted for several months each year. All this evidence suggests that early humans hibernated during extreme weather conditions with limited food supply and body stores of fat, depressing their metabolism to survive for longer periods recorded by disruptions in bone growth. Also, the pattern of damage to bones of these ancestral humans was found to be the same as the bones of other mammalian species (cave bears) in the cave who hibernate during frigid conditions.
Significance of Hibernation
Hibernation can have many benefits if induced in humans as studied in animals. For instance, squirrels, when hibernating, lower their body temperature and metabolic rate. During this phase, the brain of squirrels undergo stress and has demonstrated incredible neuronal plasticity. In hibernation, their neurons shrink, but they have neurons better than before, pointing to neuronal recovery when they are aroused. It is thought that during hibernation, there is a buildup of the protein tau in their brain, which is also found in the brains of people with Alzheimer’s disease. Somehow when squirrels were aroused, they could remove this protein from their brain. Research in this area of hibernation and the ability to scour protein from the brain that causes Alzheimer’s disease can provide humans with an innovative treatment for neurodegenerative diseases. The other use of hibernation is that the cells and tissues can survive extremes of cold temperatures, which in the case of humans can benefit from transporting or preserving human tissues for transplantation.
Hibernating animals preserve muscle mass and prevent muscular deterioration after long periods of inactivity. Thus, it can be used in bedridden humans or to treat muscular disorders to avoid deterioration that normally occurs in bedridden patients.
Similarly, hibernation can be used in humans to treat degenerative diseases of bones because hibernating animals lack bone degeneration. Thus, it could lead to new ways of treating such disorders that cause bone degeneration. You might have heard about ‘sleep our way to Mars’ that was suggested by NASA. Traveling to far-off galaxies can be made much easier by inducing a temporary state of hibernation in humans because, in this case, the travelers can go through with a limited supply of oxygen and food.
How Hibernation Delays Aging?
Aging is an inevitable process, but it can be delayed by controlling or working on the factors that promote aging. Aging is a complex process regulated by multiple biological, physiological, environmental, and biochemical factors. For example, it is seen that some mammalian hibernators have lived up to 9.8 times greater than their average lifespan. This longevity is attributed to their ability to hibernate. A common factor among these mammals is their ability to reduce metabolic rate and initiate cytoprotective responses vital to their survival. These processes enable them to survive and make their bodies rely on the limited amount of energy or fuel stored internally.
There are many things that people are using to delay the aging process including, supplements, drugs, stem cells, calorie restriction, and much more. The effectiveness of the results for all these interventions varies from individual to individual. Since metabolic rate correlates with aging that is depressed during hibernation makes sense that hibernation can promote longevity. Hibernation also activates certain cytoprotective or antioxidant defense mechanisms in the body that also work to delay aging.
A new study detects the slowing down of aging by hibernation involving the investigation of certain epigenetic markers (methyl groups) that usually bind to DNA (genome) and alter its activity. The scientists studied slowing down aging by hibernating in a group of 73 female yellow-bellied marmots (rodents that hibernate for up to eight months per year). The researchers used an epigenetic pacemaker in marmots that tracks down epigenetic changes over time. The results showed by EPM (epigenetic pacemaker) were that there was a rapid change in epigenetic age until marmots reached two years. But after reaching adulthood, the changes in epigenetic age were slower, similar to that observed in humans older than 20 years. Thus, the results clearly showed that biological aging slows down during hibernation.
Effects of Lowering Body Temperature as in Hibernation
Just like the body temperature drops during hibernation, lowering body temperature with improved survival rates has been seen in many instances. For example, in 2000, a Swedish radiologist was submerged in a frozen stream in Norway for 80 minutes to rescue a person. Although her breathing and heart stopped, doctors at Tromso University Hospital recorded her body temperature as 13.7C, the lowest ever recorded in a victim of accidental hypothermia. By all these findings, but after spending ten days in intensive care and rewarming, she woke up and recovered fully. She survived over an hour of freezing temperature that somehow cold preserved her life.
The benefits of cold or low temperature had also been seen in the Napoleonic era when the wounded infantrymen who were kept in cold had better survival rates than those kept near to fire in warm tents. Therapeutic hypothermia is also used in hospitals to manage cardiac arrest or neonatal asphyxia. It is neuroprotective and reduces mortality. Lowering body temperature reduces metabolic activity, around 5-7 percent for every degree dropped. It in turn, reduces the demand for oxygen and other nutrients making the tissues survive that are starved of oxygen due to cardiac arrest or ischemia. This mechanism can lead to the concept that if body temperature is reduced, then biological processes in the body would be suspended. Like a stopped clock, all the internal systems would stay intact but stationary, and with a little heat or temperature rise, the body can get into motion again.
Hibernation and Humans
The fact that hibernation promotes longevity can delay aging effects in humans if researchers successfully induce hibernation in humans. For the time being, it is just a theory based on the studies carried out on animals. Hibernation in humans is not easy because talking about hypothermia; the human body is made to maintain a body temperature of 37C. If it is dropped too low, say at 25C, the heart can stop, and warming up again can have other harms like kidney damage. But the 98% of shared genes of humans to hibernators, that is, Madagascan lemur, gives hints that humans may have retained some of the abilities of hibernation, like activation of protective mechanisms against cold, starvation, or inactivity. Another challenge incurred in human hibernation is to find a mechanism like in animals that prevent blood from clotting after its flow is slowed down on entering the hibernation phase. Humans are at risk of developing blood clots with hours of inactivity. Studies published in 2014 in PLOS by the Hennings team described how hibernators regulate platelets to alter the coagulation tendency of slow-moving blood. But the team is yet to prove their studies, and if they succeed it will be a breakthrough in managing many disorders like cancer, heart diseases, and severe blood loss.
Researchers are also looking to find a cure for obesity and insulin resistance by studying the hibernation mechanisms in bears. The bears eat twice their body weight in preparation for hibernation, and they usually respond to the insulin that prevents the breakdown of fats. But when they hibernate, a drop in cellular sensitivity to insulin is seen, allowing them to break stored fatty acids for energy production. Upon resuming the hibernation phase, the bear shows normal insulin functionality as seen before hibernation. Studying the pathways involved in this process can provide an ultimate cure to type 2 diabetes in which accumulated fats induce insulin resistance.
In 2006 Mitsutaka Uchikoshi, a Japanese citizen became the first documented case of human hibernation. He survived after staying in an unconscious state for 24 days in an accident while trekking. He suffered a broken pelvis and recovered thoroughly afterward. In 2010, 64 percent depression in metabolic rates was seen by entering into advanced meditative states. The study was carried out by a team led by MS Malhotra from the Medical Commission of the Indian Olympic Association and Herbert Benson from Harvard Medical School.
While there are many research studies on animal hibernation, prolonged hibernation in humans is still questionable. Recent advances in technology and genetic and pharmacological tools have already shown the potential to manipulate or induce sleep. However, the true hibernation of humans still relies on the ancestral theories of hibernation. To safely induce hibernation to delay aging or promote longevity, there is still much room to thoroughly study the pathway and circuits involved in preserving the bodily systems while hibernating. It requires regulation of sleep, metabolism, blood clotting, immune system, nervous system majorly. The body and all systems will be in a standstill state, and upon awakening from hibernation, will be the same as before going into hibernation.
However, there is a hope with the research studies ongoing on this subject that human hibernation will be a reality in the future. It will then be used to delay aging by depressing metabolism and activating other cytoprotective mechanisms that reduce oxidative stress (one of the common factors in aging).