Living against the biological clock: The role of sleep, meal timing and circadian patterns in adiposity and dietary intake in young adults

Abstract

Our daily lives are controlled by three clocks: the sun clock, the biological clock, and the social clock (local time). The sun clock gives our daily life a 24h periodicity (mainly through light- dark cycles), while the biological clock (a popular name for the circadian system) orchestrates physiology and behavior based on the sun clock. In this regard, the biological clock prepares the body to eat, stay awake and physically active during the day (sunlight), while it prepares the body to sleep and fast at night (darkness). Therefore, the synchrony between clocks is essential for health.

However, with the introduction of electric lighting, humans can select their light-dark cycles and usually prolong wakefulness far into the night. This allows people to eat, stay awake, or even work when they would normally be sleeping, altering the synchrony between the biological clock and the sun clock (also known as circadian misalignment). Interestingly, as people have begun to live against the biological clock, the prevalence of obesity has increased worldwide, suggesting that circadian misalignment can also play a role in obesity. As such, the evidence has made it clear that sleep and meal timing are determinants of body weight and dietary intake. Furthermore, recent studies are beginning to reveal that the synchrony between sleep and meal timing could be even more relevant and influential in obesity and dietary intake.

Therefore, in this Thesis we aimed to study the influence of sleep and meal timing on body weight and adherence to healthy dietary patterns in young adults. Additionally, we investigated whether the alterations of the circadian pattern of temperature and energy intake were associated with body weight and adiposity in young adults. It is worth noting that young adults are very likely to suffer from circadian misalignment, and in addition, the transition between adolescence and adulthood has been identified as a period of increased risk for development of overweight, sedentary lifestyle and poor diet quality. Therefore, studying this population is of special interest since the findings contribute to future recommendations for the prevention of obesity in young adults, which certainly also apply to the general population.

Overall, our results suggested that in young adults i) discrepancy in sleep and/or meal timing on weekends vs. weekdays, ii) misalignment between sleep and meal timing, iii) poor sleep quality, as well as iv) alterations in the circadian pattern of temperature and v) low fragmentation of the circadian pattern of energy intake were determinant factors in obesity and unhealthy dietary intake.

According to our findings, maintaining regular sleep and meal schedules during the week, and matching sleep timing behavior with dinner timing could contribute to align behavior with circadian physiology. This, in the long run, could have a beneficial impact on weight and adiposity, which is especially important for young people who are susceptible to have a late chronotype and thus, are more prone to suffer from circadian misalignment. However, these results could also be useful among other collectives, such as teenagers, adults with a greater tendency of obesity or those who live in countries where late-dinner is common.