Lifestyle+ — 24

Arestful good night’s sleep can provide a magical sense of restoration and wellness. In recent years, we’ve gleaned a better understanding for what’s going on while we sleep, and why it’s so essential to our health. During sleep, metabolic waste products are transported out of the brain by cerebrospinal fluid and interstitial fluid. Not to be confused with our lymphatics and lymph nodes in the rest of our body, the brain’s glymphatic system is a network of vessels that interacts with and complements the cerebrospinal fluid’s capability to clear the waste products. During non-rapid eye movement sleep, there are large waves of cerebrospinal flow. Impaired waste clearance has been documented in young, healthy individuals with a single night of sleep deprivation, as reflected by increases in the beta-amyloid protein seen in brain imaging. Marked accumulation of this protein is considered a precursor to the development of Alzheimer’s disease. The dynamics for cerebrospinal and interstitial fluid flow is influenced by neurovascular coupling and our circadian rhythm: during the day, our waste production is high and our vascular tone is increased, whereas slow wave neuronal activity is low. These dynamics are reversed during the night. In the words of Michael Grandner and Fabian-Xose Fernandez, “Sleep is a non-negotiable biological state required for the maintenance of human life… our needs for sleep parallel those for air, food, and water.” The relationship between sleep and health outcomes is profound, including all-cause mortality, cardiovascular mortality, cancer mortality, risk of cancer, type 2 diabetes and metabolic dysfunction, immune system function, obesity, Alzheimer’s disease, hypertension, stroke, female reproductive health, and mental and behavioral health. A brief review of a few relevant studies provides a sense of the big impact. From the UK Biobank cohort, nearly ninety-nine thousand participants with a mean age of sixty-two years using sleep trackers, sleep regularity index (SRI) (probability of an individual being in the same state of sleep or awake at any two time points twenty-four hours apart, averaged over seven days) was correlated with all-cause, cardiovascular, and cancer mortality (fig. 3.9).

Figure 3.9. The sleep regularity index (SRI) relationship (adjusted hazard ratio (HR), risk metric) for all-cause (A), cardiovascular disease (CVD) (B), and cancer (C) mortality. Adapted from Lachlan Cribb et al., “Sleep regularity and mortality: A prospective analysis in the UK Biobank,” eLife 12 (November 2023): RP88359, https://doi.org/10.7554/eLife.88359.

Among nearly eight thousand participants with twenty-five-year follow-up, a sleep duration at age fifty to sixty for six hours or less, compared with seven hours or more of sleep, was associated with a 30 percent increased risk of dementia. In a randomized trial of people with habitual sleep duration of less than 6.5 hours and a BMI between 25 and 30 kg/m2, simply coaching for better sleep led to an extra 1.2 hours of sleep and a significant decrease in daily caloric intake (270 calories). A study in young, healthy men who underwent adipose tissue and muscle biopsies after one night of deprived, as compared to full, sleep showed deleterious pro-inflammatory changes of the transcriptome, proteome, epigenome, and metabolites in the blood. The loss of one hour of sleep from daylight savings time in Germany and the United States was associated with a significant rise in heart attacks in both countries for four days.