Exploring Sleepiness and Entrainment on Permanent Shift Schedules in a Physiologically Based Model

📖 Top 20% JournalFeb 7, 2012Journal of biological rhythms

Sleepiness and body clock adjustment in permanent shift work schedules

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Abstract

Average sleepiness increases during the first days of night and evening shift schedules, with higher phase differences leading to larger initial increases.

Sleepiness decreases and stabilizes after initial increases due to circadian re-entrainment to shift schedule cues.
The initial increase in sleepiness is strongly correlated with the phase difference between circadian oscillators entrained to light and those to the shift schedule.
A critical shift onset around 2300 h separates night and evening schedules, influencing circadian phase advance and delay.
Night schedules are associated with faster entrainment of the circadian pacemaker compared to evening schedules due to longer exposure to light.
The model predicts that workers may not achieve full entrainment on shift schedules without consistent sleep routines during weekends.

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The effects of permanent shift work on entrainment and sleepiness are examined using a mathematical model that combines a model of sleep-wake switch in the brain with a model of the human circadian pacemaker entrained by light and nonphotic inputs. The model is applied to 8-hour permanent shift schedules to understand the basic mechanisms underlying changes of entrainment and sleepiness. Average sleepiness is shown to increase during the first days on the night and evening schedules, that is, shift start times between 0000 to 0700 h and 1500 to 2200 h, respectively. After the initial increase, sleepiness decreases and stabilizes via circadian re-entrainment to the cues provided by the shifts. The increase in sleepiness until entrainment is achieved is strongly correlated with the phase difference between a circadian oscillator entrained to the ambient light and one entrained to the shift schedule. The higher this phase difference, the larger the initial increase in sleepiness. When entrainment is achieved, sleepiness stabilizes and is the same for different shift onsets within the night or evening schedules. The simulations reveal the presence of a critical shift onset around 2300 h that separates schedules, leading to phase advance (night shifts) and phase delay (evening shifts) of the circadian pacemaker. Shifts starting around this time take longest to entrain and are expected to be the worst for long-term sleepiness and well-being of the workers. Surprisingly, we have found that the circadian pacemaker entrains faster to night schedules than to evening ones. This is explained by the longer photoperiod on night schedules compared to evening. In practice, this phenomenon is difficult to see due to days off on which workers switch to free sleep-wake activity. With weekends, the model predicts that entrainment is never achieved on evening and night schedules unless the workers follow the same sleep routine during weekends as during work days. Overall, the model supports experimental observations, providing new insights into the mechanisms and allowing the examination of conditions that are not accessible experimentally.

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