Abstract
"Background: Biological rhythms in humans may be driven by both endogenous circadian clocks, and exogenous environmental and behavioural cycles. The timing of environmental time-cues and behaviours may align with, or be discordant with, endogenous circadian timing, potentially influencing metabolic health and dietary habits.
Methods: The overall aim of this research was to investigate the respective contributions of the circadian cycle and behavioural cycle on metabolic responses to food intake and meal timing. Two separate studies were undertaken. The first study was an 8-day in-patient laboratory protocol, with controlled sleep-wake timing, light cycles, meal timing and composition, inducing a 5 h delay in sleep-wake and meal timing to measure differences before/after the phase delay. The second study was a 14-day longitudinal observational study, with baseline assessments of social jetlag, chronotype, and dietary data recorded each day using a smartphone app, encompassing two working weeks and weekends.
Results: In the first study, melatonin gradually resynchronised to the delay in environmental/behavioural cycles (p < 0.001). In the 24-h following the phase delay, there was a significantly lower rate of gastric emptying following breakfast (p = 0.037), lower fasting plasma glucose (p = 0.003), and a higher postprandial plasma glucose (p = 0.005) and triglycerides (p = 0.003), effects which were attenuated 48-72 h after the phase delay (Chapter 3). For energy expenditure data, analysis modelling for circadian variation in underlying resting metabolic rate nullified any diurnal differences in thermic effect of feeding (p = 0.699) (Chapter 4). In the second study, >2 h SJL was associated with significantly greater energy intake after 16:00 h (p < 0.001) and a delay in timing of the first and last meals on weekends (p < 0.001) (Chapter 5).
Conclusions: A 5 h delay in sleep-wake and meal timing may acutely disrupt metabolic physiology, with varying time-courses of adaptation to a new behavioural cycle. Higher evening energy intake may partly explain adverse metabolic effects typically associated with greater social jetlag levels in previous research."