Past your bedtime? Inconsistency may increase risk to cardiovascular health

March 24, 2020

Science Daily/University of Notre Dame

Researchers found that individuals going to bed even 30 minutes later than their usual bedtime presented a significantly higher resting heart rate that lasted into the following day.

Despite increasing awareness of how critical sleep is to our health, getting a good night's rest remains increasingly difficult in a world that's always "on" -- responding to emails at all hours, news cycles that change with every tweet and staring endlessly into the blue light of cell phone, tablet and computers screens.

Scientists have stressed the importance of healthy sleep habits, recommending at least seven hours each night, and have linked lack of sleep to an increased risk in numerous health conditions, including diabetes, stroke and cardiovascular disease.

Now a new study shows whether or not you go to bed on time could also have an effect on your health. Researchers at the University of Notre Dame studied the correlation between bedtime regularity and resting heart rate (RHR) and found that individuals going to bed even 30 minutes later than their usual bedtime presented a significantly higher resting heart rate that lasted into the following day.

"We already know an increase in resting heart rate means an increased risk to cardiovascular health," said Nitesh Chawla, the Frank M. Freimann professor of Computer Science and Engineering at Notre Dame, director of the Center for Network and Data Science and a lead author of the study. "Through our study, we found that even if you get seven hours of sleep a night, if you're not going to bed at the same time each night, not only does your resting heart rate increase while you sleep, it carries over into the next day."

Chawla and his team analyzed data collected via Fitbit from 557 college students over the course of four years. They recorded 255,736 sleep sessions -- measuring bedtimes, sleep and resting heart rate. Significant increases in RHR were observed when individuals went to bed anywhere between one and 30 minutes later than their normal bedtime. Normal bedtime was defined as the one-hour interval surrounding a person's median bedtime. The later they went to bed, the higher the increase in RHR. Rates remained elevated into the following day.

Surprisingly, going to bed earlier than one's standard bedtime also showed signs of increasing RHR, though it depended on just how early. Going to bed 30 minutes earlier than usual appeared to have little effect, while going to bed more than a half hour earlier significantly increased RHR. In cases of earlier bedtimes, however, RHR leveled out during the sleep session. Circadian rhythms, medications and lifestyle factors all come into play when it comes to healthy sleep habits, but Chawla said it's vital to consider consistency as well.

"For some, it may be a matter of maintaining their regular 'work week' bedtime through the weekend," said Chawla. "For shift workers and those who travel frequently, getting to bed at the same time each night is a challenge. Establishing a healthy bedtime routine -- as best you can -- is obviously step number one. But sticking to it is just as important."

https://www.sciencedaily.com/releases/2020/03/200324131818.htm

Human daily rhythms: Clocks vs light/dark cycle

April 3, 2018

Science Daily/University of Seville

A new study analyses daily primary activities of European laborers and the sources of social synchronization.

Should people living along a meridian be doing their basic activities (sleep/wake, working, eating, leaving home/coming home) at the same time? A 'yes' is an intuitive answer since solar noon happens simultaneously along the meridian. This phenomenon helps setting clock time. Therefore, a 'yes' is also pointing to clock synchronization, irrespective of latitude.

On a second thought we may understand that nobody gets to work at, say, 8am because it is four hours before noon, the bare meaning of clock ticking 8am. Instead the decision making process (whether 8am is fine, late or early) is driven by light conditions: did the Sun already rise? The answer to this question varies with latitude and season following the natural cycle of light and dark. Therefore, if this question influences human behaviour then people living along a meridian would not be doing their basic activities at the same time.

José María Martín Olalla, professor at University of Seville, addresses this issue in a paper entitled "Latitudinal trends in human primary activities: characterizing the winter day as a synchronizer" published in Scientific Reports, the Springer Nature open access megajournal. From time use surveys in 17 European countries and 2 American countries (located from 35º to 61º latitude) he characterizes laborer's primary activities and get them positioned along the daily and yearly cycle of light and dark.

Results show up latitudinal patterns tied to the light/dark cycle with the winter terminator as a source of synchronization for daily activities of laborers. Societies memorize the shortest photoperiod (daytime) of the year, the most challenging condition in one year. Winter photoperiod decreases by two hours from 40 to 54 degree latitude.

Winter sunrise (the later sunrise of the year and increasingly later with increasing latitude) triggers human activity in the morning year round and dominates morning trends. Its fingerprint can be traced on rising times, leaving home and working start times, all of them occur earlier with decreasing latitude. From 40 to 54 degree latitude, winter sunrise delays by one whole hour, the size of a standard time zone.

Winter sunset (the earliest sunset of the year, increasingly earlier with increasing latitude, it delays another whole hour from 54 to 40 degree latitude) triggers the reverse, shutdown process and dominates evening activities like stop working, coming home or dinning.

Two overturning sequences can also be identified. The first one occur at noon where lunch times exhibit both a meridional behaviour (tied to noon) and a latitudinal trend tied to the winter sunset. In this case people advance lunch times as latitude increases foreseeing the incoming dusk while people delay lunch times with decreasing latitude as light conditions do not worsen comparatively too much.

The second overturning sequence occurs at night and indoors: TV prime time marks and bedtimes are not tied to the winter sunset. Instead, they exhibit meridional behavior or trends weakly coupled to the winter sunrise. Societies are foreseeing the uprise in the following day.

The magnitude of the latitudinal gradient which dominates human activity can be comparatively traced out by observing how the terminator sweeps Europe in winter, when morning times are relatively similar as the sunrise terminator efficiently sweeps the continent, while evening times goes step by step following the sunset terminator.

Indirectly this study also inspects the role of time zone and time advance in human behaviour. The case of France, Belgium and Spain illustrates this issue. There, clocks are set one hour ahead of their physical time zone: that is an advanced clock, not an uncommon option for local time on Earth. Despite this time marks make perfect sense when properly tested against the LD cycle. That means people offset clock advancing by delaying time schedules apparently. In so doing they kept in phase with the LD cycle. This poses no harm to population. It only jeopardizes time comparisons, most notably in Spain due to its Southwestern most location. A rule of thumb valid for comparisons (both academic and non-academic) is subtracting one whole hour. That would convert "advanced clock" reading into standard time values.

https://www.sciencedaily.com/releases/2018/04/180403090052.htm

New Evidence of Age-Related Decline in the Brain's Master Circadian Clock

July 25, 2011

Science Daily/University of California - Los Angeles

A new study of the brain's master circadian clock -- known as the suprachiasmatic nucleus, or SCN -- reveals that a key pattern of rhythmic neural activity begins to decline by middle age. The study, whose senior author is UCLA Chancellor Gene Block, may have implications for the large number of older people who have difficulty sleeping and adjusting to time changes.

"Aging has a profound effect on circadian timing," said Block, a professor of psychiatry and biobehavioral sciences and of physiological science. "It is very clear that animals' circadian systems begin to deteriorate as they age, and humans have enormous problems with the quality of their sleep as they age, difficulty adjusting to time-zone changes and difficulty performing shift-work, as well as less alertness when awake. There is a real change in the sleep-wake cycle.

"The question is, what changes in the nervous system underlie all of that? This paper suggests a primary cause of at least some of these changes is a reduction in the amplitude of the rhythmic signals from the SCN."

The SCN, located in the hypothalamus, is the central circadian clock in humans and other mammals and controls not only the timing of the sleep-wake cycle but also many other rhythmic and non-rhythmic processes in the body.

The SCN keeps the system of multiple distributed circadian oscillators in synchrony, but disruptions in the SCN lead to disrupted sleep, as well as dysfunction in memory, the cardiovascular system, and the body's immune response and metabolism.

The SCN, Block said, can be imagined as a heavy pendulum that controls many light pendulums (oscillators), with rubber bands between them.

"If the central clock weakens, it's effectively like making those rubber bands thinner and weaker," Block said. "When the SCN ages and those rubber bands become weaker, it becomes hard for the SCN to synchronize all of these other oscillators."

http://www.sciencedaily.com/releases/2011/07/110719093808.htm

Blue-Blocking Glasses to Improve Sleep and ADHD Symptoms Developed

November 14, 2007

Science Daily/John Carroll University

Scientists at John Carroll University, working in its Lighting Innovations Institute, have developed an affordable accessory that appears to reduce the symptoms of ADHD. Their discovery also has also been shown to improve sleep patterns among people who have difficulty falling asleep. The John Carroll researchers have created glasses designed to block blue light, therefore altering a person's circadian rhythm, which leads to improvement in ADHD symptoms and sleep disorders.

The individual puts on the glasses a couple of hours ahead of bedtime, advancing the circadian rhythm. The special glasses block the blue rays that cause a delay in the start of the flow of melatonin, the sleep hormone. Normally, melatonin flow doesn't begin until after the individual goes into darkness. Studies indicate that promoting the earlier release of melatonin results in a marked decline of ADHD symptoms.

http://www.sciencedaily.com/releases/2007/11/071112143308.htm