The U-Shaped Mortality Curve Epidemiological data consistently show a U-shaped or J-shaped association between sleep duration and all-cause mortality. Individuals sleeping approximately 7 hours per night exhibit the lowest risk of death.

• Short Sleep (<6–7 hours): Associated with accelerated biological aging, metabolic dysregulation, and higher risks of cardiovascular disease and respiratory mortality.

• Long Sleep (>8–9 hours): Linked to a 30% higher risk of dying compared to normal sleepers, potentially reflecting underlying systemic inflammation, frailty, or sleep fragmentation rather than a restorative benefit.

Molecular Mechanisms: "Circadian Aging" The circadian clock governs the timing of gene expression, metabolic flux, and genomic repair. Aging is associated with a dampening of these rhythms, a process termed "circadian aging".

• Transcriptomic Collapse: Mistimed sleep (desynchrony between the sleep-wake cycle and the internal clock) causes a profound disruption in gene expression. In human blood transcriptomes, the number of rhythmic genes drops from 6.4% to just 1.0% during misalignment, disrupting processes vital for chromatin modification and translation.

• SIRT1 and NAD+: The protein SIRT1 acts as a metabolic sensor, linking cellular energy (NAD+) to the circadian clock. SIRT1 activity declines with age, weakening the clock's amplitude and impairing the cell's ability to respond to stress.

• DNA Repair: Mechanisms like Nucleotide Excision Repair (NER) are "gated" by the circadian clock, peaking at specific times of day. Disrupted rhythms leave DNA vulnerable to damage accumulation, a primary driver of aging.

Physiological Maintenance: The Glymphatic System and Thermoregulation Sleep serves unique housekeeping functions that preserve longevity:

• Brain Clearance: The glymphatic system clears metabolic waste (e.g., amyloid-beta, tau) from the brain primarily during slow-wave sleep. Aging impairs this system, and sleep fragmentation exacerbates the accumulation of neurotoxic proteins associated with Alzheimer's disease.

• Thermal Regulation: Sleep is coupled with a drop in core body temperature. This cooling induces the expression of "cold-inducible" RNA-binding proteins (e.g., RBM3, CIRBP) that protect neurons and regulate circadian gene expression.

Early-Life Programming Research in Drosophila indicates that sleep patterns established in early life are critical predictors of total lifespan. Pharmacological restoration of sleep in young fruit flies extends longevity, whereas similar interventions in older flies do not, suggesting a critical window for setting lifelong temporal health.

Optimization Maintaining "temporal homeostasis" through consistent light exposure, meal timing (time-restricted feeding), and sleep schedules can reinforce peripheral clocks and potentially compress morbidity.