Every night, we close our eyes and disappear from the world we’ll wake up to. Hours pass us by while we are either falling through the sky, standing in front of our childhood home, talking to someone we’ve only seen in passing, or even racing through foreign streets in a high-speed car chase. Sleep may feel like time lost, where minutes can feel like hours and entire stories unfold beneath our eyelids. Yet, while our minds wander through magical, imaginary worlds that disappear by morning, our body is far from inactive.

Behind every dream that our mind creates is an organized biological process. Throughout the night, we cycle between four stages: three non-rapid and one rapid eye movement stage. Non rapid eye movement (NREM) progresses through light sleep to deep, slow-wave sleep, while REM is the stage most associated with dreaming. During these hours, changes occur in brain activity, metabolism, hormone secretion, and immune function. As understanding of these phases grows, researchers are drawing connections to how quickly our bodies age.

We usually understand age chronologically, based on what is written on our birth certificate. Biological age, however, reflects the changes within a smaller part of ourselves: our cells, tissues, organs, and other physiological systems. These changes can occur at the cellular level through processes such as DNA damage and cellular aging. Two people of the same age may exhibit different patterns of biological aging.

A 2026 paper published by Cliodhna Kate O’Toole and her team investigated the relationship between sleep and biological age. They used 23 biological aging clocks across different organs and molecular systems. These clocks estimate how biologically old different parts of the body appear by measuring age-related changes, such as protein levels and the presence of metabolites produced by the body’s normal metabolic processes, and then by comparing them with patterns seen at different ages. Nine of the clocks demonstrated significant nonlinear associations with sleep duration. Several of the clocks followed a U-shaped pattern, meaning that both shorter and longer sleep durations were associated with greater biological age gaps, while moderate sleep durations were associated with lower biological age gaps. The lowest biological age gaps were observed at 6.4-7.8 hours of sleep.

But what biological mechanisms might explain this relationship? A 2021 article from Judith Caroll and Aric Prather examined how sleep disturbances may influence biological aging. Disrupted sleep interferes with cells' restoration processes and contributes to processes associated with aging, including mitochondrial dysfunction, DNA damage, and telomere shortening. Telomeres, the protective structures on the ends of chromosomes, shorten, pausing cell division. Cells then fail to perform their functions and enter a state of permanent cell cycle arrest or begin to die (apoptosis), contributing to inflammation. Together, these findings suggest that the relationship between sleep and aging begins on a cellular level.

The relationship extended beyond just biological aging. Using genome-wide association study data from UK Biobank, researchers identified a relationship between abnormal sleep duration and systemic diseases. Short sleep demonstrated genetic associations with cardiovascular and metabolic conditions, such as heart failure, coronary atherosclerosis, type 2 diabetes, and more. Some neurological and psychiatric conditions included migraines, depression, and anxiety. In contrast, long sleep showed a pattern of genetic correlations with brain conditions, including major depressive disorder, schizophrenia, bipolar disorder, and ADHD. These findings suggest that the relationship between sleep and aging extends beyond multiple systems throughout the body.

As morning arrives, the worlds that our minds create beneath our eyelids slowly fade away. The dreams that we experience become small fragments that we can barely recall in the morning. Yet, beneath all of that, our bodies are still active, and our cells, tissues, and organs spent the night undergoing processes that may influence our biological clocks. While our minds escape into imaginary worlds, our bodies keep track of time.