Lifestyle+ — 25

As you can see in figure 3.10, the relationship of sleep duration to health was not linear. It may come as a surprise that beyond seven hours there were consistent signs, both acutely and during follow-up, of cognitive and mental health decline, as well as unfavorable changes in brain structure. Also consistent is the relationship of long sleep (more than eight hours) and heightened all-cause mortality (by approximately 30%) that was previously established among nearly 1.4 million people. To sum up all the studies of sleep duration and cardiovascular disease, every one hour a night decrease in sleep below the seven- to eight-hour threshold is associated with 6 percent higher risk of total cardiovascular disease. Every one hour a night increase in sleep duration above that seven- to eight-hour threshold is associated with 12 percent higher risk of total cardiovascular disease.

Figure 3.10. Relationship of sleep duration with cognition and mental health from the UK Biobank study. Adapted from Yuzhu Li et al., “The brain structure and genetic mechanisms underlying the nonlinear association between sleep duration, cognition and mental health,” Nature Aging 2, no. 5 (May 2022): 425–37, https://doi.org/10.1038/s43587-022-00210-2.

In 2017, Matthew Walker, a professor of neuroscience at UC Berkeley, sounded the alarm about the silent sleep loss epidemic as “the greatest public health challenge we face in the twenty-first century.” Even before the COVID-19 pandemic, 35 to 40 percent of Americans were sleeping less than seven hours per night on a regular basis. Intercountry differences for sleep duration were addressed by a study of 1.1 million people with an aggregate of thirty-four studies of participants using sleep trackers in the Netherlands, the UK, and the United States. Americans got the least sleep. That same study zoomed in on insomnia symptoms by age. In people over sixty-five years of age, 15 percent had difficulty in initiating sleep, 20 percent had difficulty in maintaining sleep, and 34 percent had early morning awakenings.

The problem in older adults with sleep is threefold. There’s less deep sleep, the precious non-rapid eye movement sleep substage that promotes health across all systems, along with memory and cognition. By late age forties, it’s already lowered more than 60 percent than in a teenager; by age seventy, it’s down 80 to 90 percent. There’s also more fragmented, disrupted sleep—reduced sleep efficiency—the percent of time asleep while in bed. On top of these disruptions, there is regression of circadian timing that leads to earlier bedtimes and earlier awakenings.