Loneliness is painful, but it may serve an important purpose.
New research explores how loneliness affects the brain and body in humans and animals.
The findings reveal that loneliness triggers primal responses designed to get us to seek out social connection.
Key facts:
- Loneliness activates threat responses in the brain and body to motivate us to reconnect. But long-term, it can harm health.
- Loneliness alters sleep, attention, hormone levels, immunity, blood pressure, and heart function in humans and animals.
- It makes animals more alert to danger but also depressed. Similar brain changes happen in lonely humans.
- Loneliness has deep evolutionary origins across species tied to survival. Animals isolate to avoid disease or attacks.
Source: Perspectives on Psychological Science 2015 Mar 10(2):202-212
The Pain of Feeling Lonely
Feeling lonely hurts. Most people have experienced loneliness at some point when they felt isolated or disconnected from others.
Loneliness makes us feel anxious, sad, and uneasy.
New research suggests these painful feelings serve a primal purpose—to alert us that our social bonds are at risk so we fix them.
Loneliness triggers ancient threat responses in the nervous system that once protected us in the wild.
Our early human and animal ancestors relied on group living for safety.
Getting separated from the group and becoming isolated left them vulnerable to predators and attacks.
Being on the social outskirts was dangerous.
Animals that developed rapid detection of and responses to social isolation were more likely to survive and pass down this trait.
As a result, the brain evolved to see loneliness as a threat and kick into self-protection mode when it strikes.
This helped early humans rapidly reconnect with the group to stay alive.
While this response was useful in the past, it can cause harm if loneliness becomes chronic in our modern world.
How the Brain Reacts to Loneliness
When people feel socially disconnected, it activates threat responses in parts of the brain. This puts the body on high alert to danger.
Lonely people become hypervigilant for social threats in the environment.
Unconsciously, the brain tries to protect you by making you anxious, sad, and yearning for closeness.
This motivates you to seek out companionship. Loneliness also shifts attention toward self-preservation and away from others’ needs.
The threat response affects many bodily systems tied to health and wellbeing:
- Immune System: Loneliness weakens immunity and inflammation defenses. This increases risk of viral infections and chronic disease long-term.
- Cardiovascular System: Blood pressure rises and blood vessels constrict to prepare for action. But chronic activation strains the heart over time.
- Endocrine System: Corticotropin-releasing hormone and cortisol “stress hormones” increase. This alters metabolism, digestion, mood, and sleep cycles.
- Brain Activity: The brain stays vigilant to threats even during sleep. Lonely people get less restorative deep sleep.
Lonely humans and animals both show these physical changes.
Hypervigilance and Depression
Loneliness focuses attention on self-preservation.
Socially isolated animals become more cautious and alert to threats in their environment.
Lonely humans also unconsciously monitor more for social rejection, criticism, and exclusion.
This social hypervigilance is anxiety-provoking.
Over time, it leads to feelings of sadness, depression, and despair.
Withdrawal and isolation can result, worsening loneliness.
These depressive responses to loneliness take place in animal brains too.
Studies find lonely mice and prairie voles show more depressive behaviors.
Their brains reveal lower serotonin, dopamine, and oxytocin—neurochemicals that regulate mood and social bonding.
This reflects similar biology underlying depression and social isolation in humans.
Disrupted Sleep Patterns
Sleep is disrupted by loneliness in both animals and humans.
One reason may be to remain alert to possible threats at night when alone and unprotected.
Lonely mice show less slow-wave sleep, which is deep and restorative.
People feel lonely at bedtime report lower sleep efficiency and restless sleep.
They have more nighttime awakenings and daytime fatigue.
Loneliness continues to impair sleep quality even after taking depression, stress, and other factors into account.
Overactivated Stress Response
Loneliness activates the hypothalamic-pituitary-adrenocortical axis, the brain system governing the stress response.
This leads to elevated cortisol, the primary human stress hormone.
High cortisol increases glucose synthesis to help you react to threats.
But when activated chronically, it suppresses immunity, digestion, and reproductive hormones.
Studies confirm both lonely humans and separated animals like monkeys and prairie voles have higher daily cortisol levels.
Lonely humans also have lower sensitivity to cortisol feedback signals that normally shut off the stress response.
This indicates overactivation of the HPA stress axis.
Cardiovascular Strain
Cortisol release in lonely people also constricts blood vessels and raises blood pressure. This primes the body to cope with threats.
But if sustained, high blood pressure damages arteries, the heart, and kidneys over time.
Studies find lonely humans—even young adults—already show stiffening of arteries and impaired vascular function.
Loneliness predicts development of high blood pressure years later.
Isolated monkeys and rodents also have elevated blood pressure responses to stress compared to socially housed pairs.
Weakened Immunity Research in animals and humans finds that loneliness weakens resistance to viruses and cancer.
Lonely people have poorer antibody responses to vaccines and more trouble controlling viral load after exposure.
Gene expression analysis reveals chronic loneliness impairs the front-line white blood cells that fight infection.
This reflects conservation of a protective response.
When isolated from their group, animals limit aggressive immune function to avoid autoimmune reactions.
But this suppression allows more viral replication.
While temporary isolation protects the individual, long-term isolation increases susceptibility to infectious disease in social beings.
Evolutionary Roots
The acute pain and altered physiology of loneliness represent an evolutionary adaptation.
Being on the social periphery meant higher risk of attacks from predators or hostile conspecifics.
Animals that reacted strongly to isolation were more motivated to rejoin the group.
Staying bonded improved survival and reproduction.
Responses that aid short-term preservation—like fight-or-flight reactions and sickness behavior—helped isolated animals survive just long enough to reunite with their group or find a new partner.
But these responses accumulatively damage health if loneliness becomes chronic.
Most mammal brains are wired to find separation from close companions distressing.
Even when housed with others, animals prefer to spend time with their mating partners, close kin, or familiar friends.
When they lose their preferred companions, it feels threatening.
Neurochemically, losing a pair bond affects prairie voles like losing a child affects human mothers.
The pain of loneliness differs based on bonding habits.
Solitary species like montane voles do not appear to suffer when isolated as they have little social contact normally.
But pair-bonding prairie voles show depression-like behavior and high stress hormones when separated from mates.
This parallels the loneliness humans feel after losing close loved ones.
The distress call young animals make when separated from caregivers also reflects adaptive responses to isolation present across mammals.
Baby rats separated from mothers call in ultrasonic frequencies only other rats can hear.
This summons her back to the nest. But it also risks attracting predators.
As a tradeoff, species like baby Komodo dragons never vocalize when isolated because their cannibalistic parents pose a mortal threat.
The roots of human loneliness trace far back across our shared evolutionary lineage.
Mammalian brains evolved to motivate us through pain signals to restore vital social bonds.
Loneliness alerts us to mend friendships, reconnect with community, and seek companionship before despair sets in.
Heeding the call quickly can prevent isolation from devolving into chronic loneliness that sickens the mind and body.
Recognizing loneliness as an inherent prod toward reattachment may help us override its primal reactions with more compassionate responses.
Human Solutions to Loneliness
Loneliness serves the evolutionary purpose of spurring social reconnection in the short-term.
But prolonged isolation can lead to a self-reinforcing loop of negative thinking, poor health, and withdrawal if left unaddressed.
Besides reaching out to revive old relationships, experts suggest several strategies to alleviate chronic loneliness:
- Get moving (exercise) – Regular exercise benefits mental and physical health, relieves stress hormones, and can be done with others to boost social bonding. Even individual activities like walking improve mood.
- Improve sleep habits – Prioritize sufficient sleep and practice good sleep hygiene to reduce loneliness-related fatigue and depressed mood.
- Challenge negative thinking – Reframe ruminative thoughts about being unlikeable or uncared for. Remind yourself loneliness is a natural signal to seek company, not a reflection on your worth.
- Try therapy – Cognitive-behavioral approaches help address thought distortions that intensify loneliness. Group therapy facilitates connections.
- Volunteer to help others – Reaching out altruistically combats social isolation and builds community bonds. Helping people, animals, or causes creates purpose and self-worth.
- See a doctor – Discuss any chronic loneliness concerns with your physician. Rule out underlying medical conditions like inflammation, pain, mobility issues, hearing loss, or depression that may contribute.
The painful signal of loneliness serves like hunger or thirst—to alert us to nourish vital social needs.
With self-compassion and help from others, we can satisfy the primal drive for companionship in healthy ways.
Reaching out bravely transforms loneliness from a threat cue to a catalyst for growth.
References
- Study: Loneliness Across Phylogeny and a Call for Comparative Studies and Animal Models
- Authors: John T. Cacioppo et al. (2015)
