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Aging doesn't just dull our memories; it destabilizes our emotions while further impairing sleep quantity and NREM sleep quality. Emotional dysregulation and anxiety symptoms in older adults accelerate their risk of cognitive decline and dementia, yet the underlying mechanisms remain largely unclear. In young adults, reductions in deep sleep, specifically the loss of slow wave activity (SWA) during non-REM sleep, impair the brain's ability to regulate anxiety overnight. This raises a testable hypothesis: Does age-related decline in SWA contribute to increased anxiety symptoms in older adults? We test this hypothesis in 61 cognitively healthy older adults (>65 y) experiencing varying levels of anxiety. Each participant underwent polysomnography-recorded sleep in the lab, followed by a structural MRI the next morning to assess atrophy in anxiety-sensitive brain regions. A subset of 24 participants was tracked longitudinally over 4 ± 2.02 years. The findings were consistent. Greater impairment in nighttime SWA predicted higher next-day anxiety in older adults, both at baseline and at follow-up. Brain imaging revealed the mechanism: atrophy in key emotion-processing regions was associated with reduced capacity to generate robust slow waves needed for overnight anxiety regulation. Critically, mediation analysis showed that impaired SWA fully accounted for the relationship between regional atrophy and disrupted overnight anxiety regulation. These findings suggest that even in the presence of age-related brain atrophy, intact SWA may preserve emotional stability by rescuing the brain's nightly emotional recalibration process, protecting mental health in aging populations.