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Morning Shift-Start Times and Nighttime Sleep Length - The 70% Rule


What is the best shift-start time? It’s a question that we are often asked at CIRCADIAN by both shift workers and their managers. Many of these conversations are initiated by managers who are concerned that their start time might be too early.

In many cases, but especially when working rotating 12-hour shifts, shiftworkers prefer as early a shift-start as possible - for example a 6am/6pm start rather than a 9am/9pm start. Shift workers like it because on the day shift, it maximizes the length of their evening at home after work. And on the night shift, it allows them to get home and in their beds earlier after work.

While there are certainly some advantages to early shift-start times, there are many factors involved in choosing the best start time for your operation. For example, one study that looked at the relationship between shift-start times and nighttime sleep duration on day shifts, found that shift workers could be sacrificing sleep and safety by starting too early in the morning.

The Study

In the study, 46 male train drivers with an average age of 46.5 years old were recruited to participate in a diary study for 14 consecutive days with questions about their sleep and working hours. During the course of the study, shift-start times ranging from 3:00am to 12:00pm (noon) were recorded (Ingre M, et al. 2008).

The longest sleep duration of approximately 8 hours occurred before shifts that started at approximately 10:00am. The shortest sleeps were found for shifts that started before 4:30am and were approximately 5 hours in length.

70% Rule

The study concluded that delaying shift-start time progressively later between 4:30am and 9:00am had a strong impact on sleep length, with 70% of the extra time being used for sleep. Interestingly, delaying the shift-start time past 10:00am had little effect on sleep opportunity.

Assuming that this data from railroad workers can be carried over to other shift workers, these results have important implications for the construction of shift schedules. If 70% of the extra time acquired by delaying a shift-start time is used for sleep, it means that a shift worker that starts at 7:00am would get 84-minutes of more sleep than a worker that starts at 5:00am (i.e., 70% of 120 minutes).

Before You Change Your Start Times…

Before you start scheduling all your morning shift-start times for 10:00am to maximize nighttime sleep for the day shift, you need to take into account several factors:

  • How will moving up shift-start times affect the night-shift crew?  Will it compromise their sleep or safety? For instance, extending the night shift end-time from 6am to 10am could cause significant deterioration in alertness and performance, which would eliminate many or even all the safety gains from delaying the morning shift start time. After all, research on daytime sleep after working a night shift has shown that later bedtimes (like 10 a.m.) in the morning lead to reduced sleep duration compared to earlier bedtimes (like 7 a.m.) (Akerstedt T, 1995).
  • And how will the day-shift feel about getting home later in the day?  Will it take time away from their families? Will it impact their evening activities, such a family dinners, religious services or community events? Will it change their commute time?

As you can see, a good schedule needs to take many factors into consideration – ranging from the biocompatibility of the schedule to employee preferences. Only by taking into account many different factors can you find the best shift start time and schedule for your operation.


Ingre M, Kecklund G, Akerstedt T, Soderstrom M and Kecklund L. Sleep Length as a Function of Morning Shift‐Start Time in Irregular Shift Schedules for Train Drivers: Self‐Rated Health and Individual Differences. Chronobiology International, 2008, Vol. 25, No. 2-3 , Pages 349-358

Akerstedt T. Work hours, sleepiness and the underlying mechanisms. Journal of Sleep Research 1995; 4(Suppl.2):15-22.



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