“Early to bed, early to rise makes a man healthy, wealthy and wise.” While the common saying may be cliché, a new study may provide scientific support for the importance of sleeping and waking early.
The term “chronotype” describes one’s tendency to sleep at particular times. For example a person with a late chronotype will usually sleep late and wake up late while an early chronotype will do the opposite. A person’s chronotype is largely regulated by his or her circadian rhythm, the 24-hour physiological clock that controls body temperature and hormone production as well as the sleep-wake cycle. In extreme cases, having morning and evening chronotypes may cause a two-to-three-hour shift away from the body’s normal circadian oscillations.
Recently, scientists have uncovered suites of genes called clock genes that regulate the body’s circadian rhythm.
Vsevolod Y. Polotsky, associate professor in the department of Pulmonary & Critical Medicine at the Hopkins School of Medicine, explained the mechanisms that regulate sleeping habits.
“Circadian rhythm is regulated by the light-sensitive melanopsin receptor in the retina which transmits information to the suprachiasmatic nucleus (SCN) of the hypothalamus, which is the whole body circadian pacemaker. SCN regulates the pineal gland which secretes melatonin which [then] regulates sleep onset,” Polotsky said.
All of our tissues have their own circadian clocks, and the clock gene regulates each of them. In addition to this genetic regulation, our circadian rhythms can also be influenced by environmental factors, including stress, caffeine and lighting conditions.
In the study, conducted at the Korea University School of Medicine, the researchers first sorted 1,620 middle-aged individuals into two cohorts: those with a late chronotype and those with an early chronotype. They used the “Morningness-Eveningness Test” to evaluate their subjects’ chronotypes.
The research team then analyzed other characteristics of the participants, including exercise, smoking and alcohol use. They also recorded participants’ blood pressure, height, weight, BMI and gender. They then used biochemical tests to measure metabolic factors such as plasma glucose, serum triglyceride, high density lipoprotein cholesterol and insulin levels in the blood. Lastly they looked at the body composition of each participant, focusing on visceral fat area and abdominal adipose tissue fat area.
The blood analysis, body composition measurements and demographic characterization provided a data set from which the researchers could study the relationship between sleeping late and metabolism.
The study found that people who slept late had a higher incidence of metabolic disorders. Specifically the team found that the participants with evening chronotypes had a higher incidence of diabetes, Metabolic Syndrome (a set of symptoms including high blood pressure and high blood glucose, among others), sarcopenia (muscle loss) and visceral obesity (increased visceral fat levels).
“What makes this study strong are the outcomes. Acute phenomena such as the accumulation of visceral fat and loss of muscle mass suggest a long standing pattern of metabolic change due to a daily mismatch [of circadian rhythm],” Jonathan Jun, associate professor in the department of Pulmonary & Critical Care Medicine at the Hopkins School of Medicine, said.
While the study highlights the correlation between a late chronotype and metabolic deficits in the body, scientists do not know whether this is a causal pathway. Does sleeping and waking up late lead to metabolic problems? Or do behavioral problems such as smoking and a sedentary lifestyle cause us to sleep later and then develop metabolic problems? Perhaps cognitive problems lead to behavioral problems, which then lead to sleep problems such a late chronotype. In any case, Jun notes, irregular sleep patterns, behavioral and lifestyle problems and cognitive problems are all interrelated.
What biochemical changes could explain these metabolic disorders in late sleepers?
“There all kinds of known pathways involving cortisol, catecholamines, non-esterified fatty acids that regulate glucose levels in the body,” Jun said. “Esterified fatty acid levels have been shown to be elevated in people that sleep less.”
Both Jun and Polotsky agree that the metabolic problems associated with sleep loss — such as high-calorie diet preferences, decreased energy expenditure, a sedentary lifestyle, obesity and diabetes — could be results of eating later in the evening.
“[By eating later] it might be that you are introducing calories when the normal ebb and flow of cellular transcription is not attuned to receiving calories,” Jun said.
In order to further investigate the sleep-metabolism paradigm developed in the study, Jun said he would try to adjust the habits of poor sleepers, ensuring that they receive adequate, regular amounts of sleep. He would then look for changes in their pain sensitivity, mood and metabolism among other characteristics.
Overall Jun would try to determine whether receiving an adequate amount of sleep decreases the risk of cardiovascular and metabolic disease.
“The million-dollar question,” he said, “is if we were able to change evening people to morning people, can we reverse these [metabolic] problems?”
