Modeling the Homeostatic ‘Process S’ to Assess Sleep Misalignment and Excessive Sleepiness in Shift-Workers

Aims: To quantify and model mathematically the effects on the subjects’ sleep-wake cycle, alertness, and performance of the sleep phase shift due to working at times of the day when sleep propensity is high.
Methodologies: Thirty-seven police officers working on a fast counterclockwise schedule, filled for 25 consecutive days a self-administered questionnaire about the previous night sleep, subjective fatigue, sleep attacks, errors. For each subject, the homeostatic process was computed according to the Borbély-Achermann model. Night sleep decreased progressively during the shift schedule, with effects on sleep pressure accumulation and the subjective feeling of fatigue increasing exponentially in the first four shifts to decrease during the 60-hrs off-duty.
Results: Sleep proved delayed starting from the schedule beginning, with marked differences between the morning and night shifts in the subgroups of subjects who voluntarily did/did not compensate for the sleep deprivation by morning or afternoon naps. Sleep attacks occurred mainly during the night-shift, while errors were more frequent in the morning- and night- shifts.
Conclusion: The temporal misalignment with the circadian drive of the working schedule is a primary cause of sleep disruption, with excessive sleepiness and predictable negative effects on alertness, attention and performance. The available model may help define schedules compatible with the physiological sleep-wake cycle in various workplaces.