Why Do We Sleep? A Physicist Explores One of Science’s Greatest Mysteries

Eric Hedin is a professor of physics who also teaches chemistry and astronomy at various universities throughout California. Biology and medicine are not his fields of expertise, but one morning, after a long night's sleep, he woke up with a persistent question: Why do we sleep?

While drinking his morning coffee, he made a simple calculation. A person who enjoys a long life will spend roughly 26 years sleeping. 

If life developed purely through chance and natural selection, what sense does it make for such a prolonged state of vulnerability to evolve? Sleep places humans at significant risk. While asleep, a person cannot defend themselves from predators, protect their possessions, or respond to danger. So what purpose does it serve?

Hedin searched through books and consulted university colleagues. The answer he repeatedly encountered was that sleep remains one of science's greatest mysteries.

An Unanswered Question

In an article on the subject, Hedin cites researchers who continue to ask: "Why would evolution favor species that spend one-third to one-half of their lives asleep?"

Consider giraffes, which rest only for brief periods each day. Horses also sleep very little. Scientists once believed that sleep evolved primarily to conserve energy and reduce food requirements. However, later research showed that the body continues to consume significant energy during sleep as it carries out numerous biological processes.

One researcher summarized the problem this way: "Unfortunately, evolutionary theories are difficult to prove, leaving us without an answer as to how sleep originated in the first place."

Looking Beyond Survival

Hedin concluded that attempting to understand sleep solely through the lens of evolutionary survival may actually hinder our understanding of it.

From an immediate survival perspective, sleep appears disadvantageous rather than beneficial. Yet when viewed in terms of human well-being, mental health, personal development, and long-term flourishing, its importance becomes far more apparent.

According to Hedin, the tendency to view every biological feature solely as part of a struggle for survival can obscure the deeper purposes of many human traits.

What Sleep Actually Does

Modern research has revealed numerous benefits of sleep.

Psychological studies indicate that during sleep, the brain organizes sensory information and thoughts gathered during waking hours. It sorts experiences, determines what information to store, and forms connections between new memories and existing knowledge.

Dreams may even play a role in this process.

These functions contribute significantly to emotional stability, mental clarity, learning, creativity, and overall well-being.

According to Hedin, these benefits relate more to the quality of human life than to immediate survival competition.

The Challenge of Complexity

Hedin argues that the biological mechanisms responsible for sleep and wakefulness are extraordinarily complex.

He writes: "Since the regulatory processes that cause sleep and the return to wakefulness involve the coordinated activity of numerous brain structures and neurotransmitters, calling this an irreducibly complex system seems like an understatement."

His point is that sleep alone would not be enough. A creature would also need a highly sophisticated mechanism for waking up safely and reliably.

Imagine, he suggests, an organism that somehow evolved the ability to sleep but lacked the equally complex systems required for awakening. Such a creature would be unlikely to survive for long.

According to Hedin, sleep, wakefulness, and the body's adaptation to both states appear to function as an integrated system whose components must work together.

Beyond Reproduction

Hedin further argues that many aspects of human life pose challenges for purely evolutionary explanations.

Natural selection focuses on traits that increase reproductive success. Once an individual has successfully produced offspring, many aspects of later life — such as happiness, fulfillment, comfort, and quality of life, do not directly influence reproductive outcomes.

From this perspective, Hedin asks why so many human characteristics appear geared toward enhancing life beyond mere survival and reproduction.

A Different Perspective

Hedin views sleep as one example among many features of human existence that point beyond simple biological competition.

In his perspective, qualities such as well-being, emotional richness, intellectual growth, and spiritual fulfillment are gifts that cannot be adequately measured through the framework of natural selection alone.

For him, the ongoing mystery of sleep illustrates a broader point: that some of the most remarkable aspects of human life may be better understood not merely as products of survival mechanisms, but as features that contribute to a richer and more meaningful human experience.

Whether one agrees with his conclusions or not, Hedin's question remains a fascinating one: Why do we spend nearly a third of our lives asleep — and what does that reveal about what it means to be human?