The effects of nocturnal life on endocrine circadian patterns in healthy adults
Introduction
It has become common that modern people succumb to a nocturnal lifestyle to create maximum time for work and leisure activities (Broman et al., 1996). The prominent characteristic of this nocturnal life was considered as staying up half the night. It was reported that “normal” average sleep duration decreased from about 9 hours per night in 1910 to about 7.5 hours in 1970 (Webb and Agnew, 1975). Recently, the research about inadequate sleep focused on adolescents who are in a critical developmental period Arakawa et al., 2001, Samuelson, 2000. In those studies, it was found that breakfast was skipped by many students who stayed up late.
Although there is no clear definition about nocturnal lifestyle, abnormality of sleep and food schedules is a common experience for humans with a nocturnal lifestyle. We carried out a nutritional survey in medical students in Yamanashi Medical University during 1997–2002. The energy intake and proportion of protein, fat and carbohydrate were similar to that reported by a national nutritional survey in the same age group. To our surprise, 26.7% of students failed to eat breakfast. Although sleep items were not included in the questionnaire that was designed for the nutritional survey, many participants informed us that they slept too late to allow time for breakfast the next morning. However, they bought much food from nearby convenience stores (open for 24 hours) to eat during the longer nightlife. The characteristics of the lifestyle of these university students included: 1) They go to bed later than midnight but do not show signs of significant short sleep duration because they also rise later in the morning. 2) They omit breakfast but take adequate energy because they eat much at night.
To date, most studies were conducted in extreme conditions, such as sleep deprivation (Spiegel et al., 1999) or fasting from food Arendt et al., 1982, Boden et al., 1996a, Eliman and Marcus, 2002. The aim of the present study was to observe the impact on human health when subjects lived in this lifestyle where sleep was not very short and energy intake was not inadequate, but the time of sleep was delayed and the food intake became irregular. As a preliminary study, we tried to assess the 24-hour patterns of relative hormone variables including melatonin, which appears to induce and maintain sleep, and leptin, which suppresses appetite and helps to maintain sleep. This experiment also measured insulin concentration, which is stimulated by plasma glucose after a meal.
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Subjects
Seven volunteers (6 male and 1 female) were recruited from the university campus at the beginning of February. They studied in the same classroom and lived nearby. Their ages ranged from 20 to 23 years (21.7 ± 1.3). Body mass index (BMI) before the study was 22.9 ± 3.0 Kg/m2, and showed no significant change after the study. All subjects were nonsmokers without a history of endocrine, liver or other metabolic disorders. We did not find anyone with an eating disorder or special diet in the
Results
Fig. 1 depicts the 24-hour patterns of plasma concentrations of melatonin, leptin, glucose and insulin in the diurnal lifestyle and nocturnal lifestyle groups.
The peaks of melatonin concentration appeared at 0300 h in both groups (P < 0.01, compared with other time points of the same group). In the diurnal lifestyle group, the melatonin concentration increased by 5.3-fold at the peak compared with the minimum concentration (1500 h). This increase returned to 3.0-fold that in the nocturnal
Discussion
Melatonin is predominantly produced rhythmically from the pineal gland. The nocturnal melatonin production is directly proportional to the length of the dark period. In humans, sleep is initiated during the rising phase of the melatonin rhythm Arendt et al., 1982, Follenius et al., 1995, Pévet, 2000. In turn, sleep condition influences the nocturnal secretion of melatonin. The decrease in the night production of melatonin in the nocturnal lifestyle group was expected. The midnight samples were
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2020, Medical HypothesesCitation Excerpt :The nocturnal lifestyle group displayed lower peak concentrations of melatonin, a hormone that regulates the sleep-wake cycle, and leptin, a hormone that inhibits hunger, than the diurnal lifestyle group. Plasma glucose increased after all meals in both groups, but glucose concentrations remained high while insulin secretion decreased in the nocturnal lifestyle group between midnight and early morning [13]. These findings suggest that nocturnal lifestyles may be associated with higher risk of type II diabetes and obesity which in turn are risk factors for many other health conditions.