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

Abstract

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.

The effects of self-selected light-dark cycles and social constraints on human sleep and circadian timing: a modeling approach

“…Variants of this model have been used to investigate: chronotype28, internal desycnchrony63 non-rotating64 and rotating shift schedules65, and age-related changes to sleep timing and duration29. This model has a close relationship with the original two-process model66, and similar results would result from considering a two-process model with the phase and amplitude of the circadian rhythm determined by an external signal gated by the eye as in the model used here.…”

Modelling changes in sleep timing and duration across the lifespan: Changes in circadian rhythmicity or sleep homeostasis?

“…This has resulted in several physiology-based models of the sleep-wake cycle [80,81,82] that focus on the principle groups of neurons believed to promote wake and sleep states and link them to homeostatic and circadian processes. In particular, the Phillips-Robinson model and its extensions have been extensively tested against human data and used to describe: the response to impulsive auditory stimuli [83]; measures of sleepiness during sleep deprivation experiments [84,85]; the effects of caffeine [86]; spontaneous internal desynchrony [87], and the effects of non-rotating and rotating shift schedules [88,89].…”

The Shift-Work Accident Rate is More Related to the Shift Type than to Shift Rotation

“…An additional reason for the increased accident rate in the morning shift might be related to the very early shift start at 06:00 h, which means an even earlier time of getting out of bed compounding the problem after a number of free days off. The allocation of work shifts and days off within a shift-work cycle would interfere with entrainment of the circadian clock (Roenneberg et al 2007;Kantermann et al 2010) and the sleep/wake pattern in these workers and hence with their adjustability to a certain work shift (Monk et al 1996;Knauth and Hornberger 2003;Smith et al 2009;Postnova et al 2012). This finding may imply that the circadian rhythm in sleepiness and alertness must be assumed to additionally modify accident risk at certain times of the day.…”

“…This pattern was based inter alia on early studies of time-zone travel showing that westward flights, compared to eastward flights, were accompanied by faster adjustment to a new time zone, due to the fact that phase delays of the internal clock are for most people easier to accomplish than phase advances (Knauth and Hornberger 2003;Sack et al 2007). These considerations may be applicable to some shift-work settings but studies clearly defining the contribution of both the direction and speed of shift rotation to shift-work adjustment are lacking (Costa 2003;Knutsson 2004;Kantermann et al 2010;Postnova et al 2012). In real life, for instance, complete adjustment to night work is rare even in permanent night shift-workers (Folkard 2008).…”