The idea of human hibernation, a science fiction staple, is now a subject of scientific exploration. This concept involves inducing a state of suspended animation, where bodily functions are slowed significantly. Such a state holds the potential to alter human capabilities, offering solutions to current insurmountable challenges. Researchers are delving into the biological mechanisms that allow certain animals to hibernate, to understand if these processes can be replicated in humans.
The Biology of Natural Hibernation
Natural hibernation allows animals like ground squirrels, bears, and bats to survive harsh environmental conditions and scarce resources. During this state, animals undergo metabolic suppression, reducing their energy use by 90-98%. This metabolic slowdown often precedes a drop in body temperature, sometimes to near-freezing levels, with reduced heart and breathing rates. The arctic ground squirrel, for instance, can lower its body temperature to -2.9°C while maintaining cellular function.
At the cellular level, hibernators protect tissues from cold damage and manage waste buildup over months of inactivity. Mitochondrial respiration is suppressed during torpor, with enzyme modifications playing a role. They also activate pathways, like those involving opioid receptors and AMP-activated protein kinase (AMPK), contributing to cellular protection and energy homeostasis. Small hibernators, such as the thirteen-lined ground squirrel, undergo structural changes in muscle proteins, maintaining myosin stability in cold environments.
Human Physiology and Hibernation Barriers
Unlike natural hibernators, humans have biological limitations preventing a hibernation-like state without severe consequences. An uncontrolled drop in human core body temperature, typically below 28°C, can lead to life-threatening cardiac arrhythmias. Extreme cold also causes cellular damage from ice crystals within tissues, disrupting cell membranes and organelles. Human red blood cells, for example, increase in elasticity and viscosity at lower temperatures, making blood circulation challenging.
Prolonged inactivity in humans, even without extreme cold, results in muscle atrophy and bone density loss, unlike hibernating animals which mitigate these issues. While animals maintain muscle and bone integrity during torpor, humans would experience degradation without external interventions. The human brain also presents a challenge due to its high and continuous energy demand. Lowering brain temperature without protection could lead to irreversible neurological damage or memory loss, as the brain struggles to maintain function and clear waste at a reduced metabolic rate.
Induced Torpor for Medical Treatment
Despite challenges, therapeutic hypothermia, a controlled, short-term state, is already a medical practice. This technique involves lowering a patient’s core body temperature to 32-35°C for a limited duration. It is primarily used in emergency medicine for conditions where oxygen supply to the brain or other organs is compromised. It is used following cardiac arrest to improve neurological outcomes and survival, and is also explored for treating acute ischemic stroke and traumatic brain injury.
Lowering the body’s metabolic rate protects by reducing oxygen and nutrient demand in organs like the brain and heart. This buys doctors time to address injury, as cooled tissues are less susceptible to damage. The process helps by inhibiting harmful excitatory neurotransmitters, blocking cell death pathways, and promoting neuron survival. While systemic complications like cardiac or hematologic issues can arise from whole-body cooling, selective brain cooling is being investigated to maximize benefits while minimizing side effects.
Hypometabolism for Deep Space Exploration
Inducing hypometabolism in astronauts, or “synthetic torpor,” is explored for long-duration space missions like a journey to Mars. This approach addresses multiple challenges for spaceflight. Reducing an astronaut’s metabolic rate would decrease life support consumables (food, water, oxygen), reducing spacecraft mass and mission costs.
Beyond resource savings, synthetic torpor could minimize psychological stress from prolonged confinement and isolation during months-long transits. Research also suggests hypometabolism could protect against damaging space radiation. Agencies like NASA fund projects, such as “Studying Torpor in Animals for Space-health in Humans” (STASH), to investigate how hibernation affects bone and muscle loss in microgravity and develop methods for inducing this state. While medical therapeutic hypothermia is short-term, the goal for space travel is stable torpor lasting weeks or months, requiring novel solutions to prevent bone and muscle degradation in zero-gravity.