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Volume 17, Number 2
February 2021
Diversity is advantageous in many health-related realms. Healthy diets typically include a variety of foods, particularly vegetables that have many different colors. Exercise that has both cardiovascular and strength components, and involves many muscle groups is desirable. One biological indicator of health and fitness is heart rate variability (variation in time between heart beats) and can be used as an indicator of stress resilience.
On the other hand, sleep is one area where uniformity is most desirable, both with respect to duration and timing. Investigators from Harvard Medical School published a paper in Diabetes Care in 2019 reporting results of an analysis of sleep patterns in over 2000 adults, followed for over 6 years. The primary statistical metric of interest was standard deviation (SD), an indicator of the degree of variability in a group of data. A higher SD means there is a broader distribution of measures, whereas a lower SD means that values are clustered closer to the mean. A higher SD for sleep duration means there is much variability in how much sleep a person gets from night to night. Similarly, a higher SD for sleep timing means that bedtimes and wake times for an individual are inconsistent from night to night.
It is well established that sleep deprivation is a risk factor for obesity, diabetes, and metabolic syndrome. Metabolic syndrome is typically defined as the clustering of high blood pressure, high blood sugar, abdominal obesity, and an abnormal lipid profile. The Harvard investigators found that every 1-hour increase in SD for sleep duration was associated with a 27% increased risk for metabolic syndrome, and every 1-hour increase in SD for sleep time was associated with a 23% increased risk. This increased risk was present even when average sleep duration was controlled in the analyses. While there are biochemical disruptions that occur with sleep irregularity, sleep variability also affects the timing of meals and eating frequency, both of which have been associated with obesity and risk of diabetes. The conclusion is that routine and uniformity in the timing and duration of sleep is most healthy.
Black participants were found to have a higher SD for sleep duration, possibly contributing to health inequities. Not surprisingly, persons with non-day work schedules, and those who describe themselves as “evening types” rather than “morning types” also had higher SDs for sleep duration and timing.
With respect to clock time, more than 2 hours of variability in sleep duration and more than 90 minutes of variability in sleep timing conveyed the most risk.
Although “variety is the spice of life” may be accurate for many areas of human functioning, it doesn’t apply to sleep. When possible, having a regular bedtime and wake time appears to be best, so that sleep timing and duration have a low standard deviation.
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