Associate Professor of Clinical Neurology, Weill Cornell Medical College
Dr. Segal reports no financial relationships relevant to this field of study.
- Appetite and weight are linked to circadian function.
- Circadian rhythms affect immune and inflammatory regulation and probably modify the epigenetic modulation of DNA.
SYNOPSIS: All living organisms have 24-hour circadian rhythms. A body of evidence is accumulating that chronic disruption of this important rhythm may result in poor health outcomes. These negative consequences of disrupted circadian rhythms might be prevented by modifying work and sleep schedules.
SOURCE: Guintella O, Mazzonna F. Sunset time and the economic effects of social jetlag: Evidence from US time zone borders. J Health Econ 2019;65:210-226.
Circadian rhythms are present in every living thing. Even simple prokaryotes, such as cyanobacteria, modulate their metabolism based on the wavelength of exposed light. In 2017, the Nobel Prize for Medicine was given to researchers who unlocked the molecular and genetic basis of circadian functions in the drosophila fruit fly. There are multiple determinants of circadian function (known as “zeitgebers,” or timekeepers), the most important of which is blue light. The supraoptic nucleus of the hypothalamus is regulated directly by the intensity and timing of exposure to light. Melatonin (the hormone of darkness) is produced by the hypothalamus and is another important circadian regulator.
Beyond melatonin, multiple hormones cycle on a circadian basis, the most important of which are cortisol and growth hormone. Hypothalamic temperature regulation also follows a circadian pattern, with a typical body temperature nadir occurring three hours before waking. Appetite and weight are linked to circadian function, including the effects of leptin (which promotes satiety) and ghrelin (which increases hunger). Circadian rhythms affect immune and inflammatory regulation and likely modify the epigenetic modulation of DNA.
Despite these factors, human society has attempted to wrestle control over the circadian clock by creating work schedules (particularly night shifts) that contradict cycles of natural light. While “jet lag” is a transient travel-related disruption in sleep-wake schedules, “social jet lag” is a more chronic process, ingrained in the habits of our daily lives.
Although the Earth’s rotation takes 24 hours, experiments in humans deprived of natural light or among the blind who cannot detect any light, show that the circadian cycle actually could extend to 24.5 or even 25 hours. Desynchronization between the circadian and “ultradian” cycles results in a disorder known as “hypernychthemeral syndrome” or “non-24,” in which small alterations in cycle length add up from day to day, resulting in significant disruptions in sleep-wake cycles. Research in this area is challenging, as subjects in an experimental “free running” paradigm must spend days sequestered in a state of constant dim light exposure, deprived of TV or any other external cue of day or night.
Time zones are one example of a human construct superimposed on the natural variations of day and night. Within any given time zone, sunset times are not constant, but rather get progressively later as one proceeds westward, with the latest sunset being at the utmost western border of each zone. Over this western time zone boundary, sunset shifts an hour earlier as the clock is turned back. Despite these sunset differences, work, school, and social schedules remain fixed, with rigid morning starting times regardless of location within a time zone. Exploiting these variations, the authors compared sleep times (using data derived from Fitbit-type devices) across each of the four U.S. time zones. Sleep was studied geographically at the county level and ZIP code level, and as a continuous variable across the time zone. Data were derived further from the American Time Use Survey (ATUS) and Behavioral Risk Factor and Surveillance Survey (BRFSS).
Living on the “late sunset” (westernmost) side of a time zone resulted in an average of 19 fewer minutes sleep per night compared to living at the easternmost sector of the next time zone. Alternatively, using eight hours as an “optimal” sleep duration, the “late sunset” cohort was 8% less likely to achieve the necessary amount of sleep.
The subjects were divided into “employed” (which included students) and “non-employed.” Both groups were affected by “social jet lag,” but in different ways. Living at the westernmost sections promoted late bedtimes, but this effect was more pronounced in the non-employed. While the employed were 34% more likely to be awake at midnight, the non-employed were 41% more likely to be awake at that hour. For individuals who started work at 7 a.m., their average sleep duration was 36 minutes shorter. Westernmost location promoted late wake-up times, particularly among the non-employed. While employed people were equally likely to be awake at 7:30 a.m. regardless of time zone location, non-employed people in the westernmost locations were 32% more likely to be asleep at that hour.
In addition to sleep times, health outcomes also were affected adversely. Individuals at the western boundary were 11% more likely to be overweight,
a difference that reached statistical significance. There were additional nonsignificant trends toward other adverse health outcomes, such as diabetes, cardiovascular disease, and breast cancer. Overall, “self-reported health status” was 2% worse with late sunsets, but this did not reach statistical significance.
The authors used “back of the envelope calculations” to estimate economic consequences. They determined that circadian misalignment increases healthcare costs by $2 billion. Productivity losses induced by the extra hour of light in the evening were calculated to total 4.40 million days of work nationwide. There was an estimated 3% decrease in income among those living on the western side of a time zone. Total economic losses were estimated to be $2.35 billion (approximately $82 per capita). The authors calculated that a one-hour increase in daily sleep increases productivity to a greater extent than a one-year increase in education.
There are increasing data showing that sleep plays a key physiological role in the “glymphatic” system of the brain, a “dishwashing” mechanism that widens gap junctions and facilitates the removal of toxins. Multiple studies indicate that high-quality sleep, with increased periods of REM and slow-wave sleep, promotes clearance of substances such as amyloid and tau proteins. Although day-to-day deficiencies in sleep duration promote cognitive loss (impairments in vigilance), more chronic cumulative sleep loss may have more permanent effects, including possibly Alzheimer’s disease.
Seasonal differences in light exposure and sleep times may provide quasi-experimental data similar to this time zone investigation. Although daylight duration varies from 14 hours in summer to only eight hours in winter, humans sleep a fixed amount of time. This effect is strongly driven by latitude and perhaps would be less pronounced closer to the equator. It is possible that bears or other hibernating animals behave in a more physiologically favorable manner, being active in summer and sleeping for long periods during the winter, allowing for a cumulative clearance of central nervous system toxins.
As this study suggests, there would be benefits from more flexibility in work schedules. Although banks or public offices may follow a strict 9 a.m. to 5 p.m. day, retail stores maintain potentially more realistic hours, shifted to 10 a.m. to 6 p.m. or possibly an even later interval. While television schedules are modified to broadcast at appropriate times across Eastern to Pacific time zones, there is no such modification of show times within any given time zone. Modern TV practices, with streaming of content and “binge watching,” would provide for more flexibility, but may have as-yet unrecognized adverse effects on sleep health.