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A similar permanence is found in the heart. For a long time, it was believed that the heart never replaced its muscle cells, but more recent carbon dating studies on genomic DNA have shown a very low turnover rate. About one percent of heart muscle cells are replaced each year in early adulthood, and this rate slows even further as we age. This means that by the time a person reaches eighty, about half of their heart cells are the same ones they were born with. This slow pace of renewal in vital organs like the heart and brain creates a biological paradox where the body is mostly new, yet its most critical command centers remain ancient. This makes for fascinating science for sleep, as we consider how the body maintains its identity through these varying cycles of decay and rebirth. Inside the quiet architecture of the skull, the brain undergoes a physical transformation that is as much about refinement as it is about loss. Starting in middle age, the overall volume of the brain begins to decrease by roughly five percent every decade. This shrinkage is not uniform, as it tends to concentrate in the prefrontal cortex and the hippocampus, areas responsible for complex planning and memory formation. But have you ever noticed how a well-worn path through a forest becomes clearer even as the surrounding woods grow thin? The thinning of the cerebral cortex, which is the outer layer of gray matter containing the bodies of your neurons, does not mean the brain is simply fading away. Instead, the brain often compensates for this loss of density by recruiting more areas to perform a single task. While a younger brain might use only one hemisphere to solve a logic puzzle, an older brain frequently activates both sides of the prefrontal cortex to achieve the same result.