Summary
The message of the following studies is simple: Creating a regular sleep schedule will likely improve your health. Develop the simple habit of going to bed about the same time most every night and getting up at the same time most every day.
The sleep regularity index
In 2017, Andrew Phillips and colleagues developed and validated the Sleep Regularity Index (SRI). It reflects the probability of a person being in the same sleep state (asleep or awake) at any two time points (say, 10 pm) for two contiguous days. An SRI value of zero reflects completely random sleep, while a value of 100 reflects a person who falls asleep and awakens at the same respective times every day.
A team of researchers from Duke University further tested the validity of the SRI using data from 1,978 participants with an average age of 69 years in the Multi-Ethnic Study of Atherosclerosis. For 7 consecutive days, participants wore wrist-mounted devices that measured physical activity, from which SRI values were calculated, and ambient light data.
Low levels of SRI predicted delayed sleep timing and increased daytime sleep and sleepiness independent of sleep duration. In addition, low SRI values predicted increased 10-year cardiovascular disease risk and greater obesity, hypertension, fasting glucose, HbA1c, and diabetes status. Yikes! Lower SRI values also predicted greater likelihood of self-reported depressive symptom severity and perceived stress. This study could not determine if low SRI values caused cardiometabolic problems or if cardiometabolic problems caused lower SRI values. Nevertheless, a low sleep regularity index might help identify persons at risk for cardiometabolic disease.
Better sleep regularity helps cancer patients
Cancer patients commonly report sleep disturbances, which reduce quality of life and may shorten survival. Researchers in Australia conducted a cross-sectional analysis of 87 patients (three-quarters female, average age 62 years) who had completed cancer treatment to determine if higher sleep regularity predicted better quality of life. Sleep regularity was quantified using the sleep regularity index (SLI). Data to calculate SRI came from wrist-worn actigraph units worn for an average of 11 days. The average cancer patient had a relatively high SRI value of 81. Participants with higher SLI had significantly higher quality of life, as well as higher physical functioning; less fatigue, nausea/vomiting, difficulty breathing, and diarrhea; and better sleep quality. Thus, greater sleep regularity may increase quality of life of post-treatment cancer patients.
Sleep regularity vs. sleep duration
Hearth authorities commonly advise Americans to get 7 to 8 hours of sleep per night. Recent evidence suggests that establishing a regular sleep schedule may create health and well-being benefits beyond those linked to 7-8 hours of daily sleep. Researchers in Boston and elsewhere used the sleep regularity index (SRI) to quantify day-to-day variation in sleep onset and awakening using data from 60,977 participants with an average age of 63 years in the UK Biobank. These researchers used the same participant data to predict the risks of all-cause mortality, cancer mortality, and cardiovascular mortality over an average follow-up of 6 years. After fully accounting for confounding factors and compared to participants in the lowest quintile (average SRI=62) of SRI scores, participants in the highest quintile (average SRI=90) had a significant 30, 24 and 32 percent lower risks of dying of any cause, cardiovascular disease, and cancer, respectively. Using the same data and comparable statistical models, researchers found that sleep duration was a poorer predictor of mortality than sleep regularity. This study suggests that falling asleep and awakening within one-hour windows at night and in the morning may help you live longer and better.
Sleep regularity and cardiovascular disease risk
Researchers in Singapore reviewed 22 articles that linked measures of sleep regularity to cardiovascular disease and dementia risk factors in adults with an average age of 50 years or more. Measures of sleep regularity included the sleep regularity index (SRI) and standard deviations of sleep duration, timing, and efficiency. Regardless of the measure used, low sleep regularity predicted increased cardiovascular risk factors, including increased body mass index, waist circumference, body fat, and fat mass, and higher likelihood of obesity. In addition, low sleep regularity predicted greater prevalence of hypertension, higher systolic and diastolic blood pressure, and higher risk of developing cardiovascular disease. Low sleep regularity predicted higher risk of developing mild cognitive impairment and dementia, increased beta-amyloid burden in the brain, and lower brain cortical thickness and gray matter volume. The researchers cautioned that the dementia-related studies were not as convincing as cardiovascular-related studies due to varying research designs and analyses of the former. Still, greater sleep regularity may be an important aspect of overall health.
Sleep irregularity and risk of dementia
The adverse effects of irregular sleep on cardiovascular disease risk may include increased risk for dementia. Researchers in Australia used data from 88,094 participants with an average age of 62 years in the UK Biobank to determine if the sleep regularity index (SRI) predicted dementia risk of 7 years of follow-up. After accounting for confounding factors, the risk of developing dementia followed a U-shaped path with significantly higher risk below the median SRI (60) and, counter intuitively, non-significantly greater risk above the median. Interestingly, further adjustment for physical activity, body mass index, and cardiovascular risk factors reduced the elevated risk of dementia below the median SRI value to statistical non-significance. Thus, markers of cardiovascular risk may lie along the same causal path between irregular sleep and dementia. A subset of 15,263 underwent brain scans to determine if brain volumes were related to SRI. The resulting U-shaped paths showed significantly lower gray matter and hippocampal volumes below the median SRI value of 60. This study provides additional evidence that irregular sleep ay be a risk factor for dementia.
Other researches in Australia tested whether device-measured sleep duration and sleep regularity would predict risk of developing dementia. Data came from 82,391 participants with an average age of 62 years who were followed for 8 years in the UK Biobank study. When plotted against sleep duration, the risk of developing dementia during follow-up assumed a U-shaped distribution, with higher risks appearing in sleep duration of less than 7 and more than 8 hours per night. But only short sleep duration predicted significantly increased of dementia. When plotted against sleep regularity (based on Sleep Regularity Index values), the risk of developing declined in a nearly linear manner as SRI values increased. The highest measured value of SRI (94.2) in this study predicted the maximum decline in risk of developing dementia (46 percent). Interestingly, the link between SRL and dementia risk was statistically significant only for participants with sub-optimal sleep duration of less than 7 hours per day. Thus, greater sleep regularity might partially temper the adverse effects of short sleep on dementia risk.
What to do
Go to sleep about the same time every night and awaken about the same time every morning.
