The impact of light intensity during night shifts on circadian adaptation, daytime sleep, and night-time alertness in healthy adults.
CQUniversity
60 participants
May 18, 2022
Interventional
Conditions
Summary
The current evidence suggests that the optimal length of a sequence of night shifts may depend on the intensity of light that shiftworkers are exposed to during the night. If so, then OHS guidelines that recommend a blanket limit on night shifts of a maximum of 2-4 in a row may inadvertently expose shiftworkers to a higher level of fatigue-related risk than is necessary in some workplaces. In this project, we will test the proposition that shorter sequences of night shifts may be suited to workplaces where shiftworkers operate in dim light and are less likely to adapt to a nocturnal schedule, whereas longer sequences of night shift may be suited to workplaces where shiftworkers operate in normal indoor light and may be more likely to adapt to a nocturnal schedule. The project will include a multiple-day night work simulation study with three conditions. The only difference between conditions will be in the light intensity during night shifts – dim (10 lux), moderate (100 lux), and normal (350 lux) – as experienced by truck drivers, hospital-based healthcare practitioners, and control room operators, respectively. The data will be used to test three hypotheses: 1. Body clock time will get progressively later, i.e., delay, with each successive night shift, and the daily rate of delay will be smallest (or absent) in the dim light condition and greatest in the normal light condition. 2. The quantity/quality of daytime sleep will progressively increase with each successive night shift, and the daily increases will be smallest (or absent) in the dim light condition and greatest in the normal light condition. 3. Night-time alertness will decline with each successive night shift in the dim light condition, remain stable in the moderate light condition, and increase with each successive night shift in the normal light condition.
Eligibility
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Interventions
The study will be conducted at the Sleep and Circadian Rhythms Laboratory at the Appleton Institute. This study will employ a between-groups design with three conditions. Each condition will include a simulated shiftwork protocol with 14 x 12-h night shifts (18:00-6:00) and an 8-h sleep opportunity each day (08:00-16:00). The light conditions during night shifts are: Dim Light (10 lux); Moderate Light (100 lux), or Normal Light (350 lux). The light conditions will be delivered using automated, broad-spectrum, ceiling-mounted LEDs (i.e., warm light). With the exception of light intensity during night shifts, the 16-day protocol will be the same for all three conditions: • Night 1. Participants will have an 8-h sleep to adapt to sleeping in the accommodation suite. • Day 1. Participants will be trained on all test battery tasks to minimise learning effects. • Night 2. Participants will have an 8-h sleep to establish baseline sleep parameters. • Day 2. Participants will have a 1-h sleep in the afternoon to assist the transition to night shifts. • Nights 3–16. Participants will work 14 x 12-h night shifts. During night shifts, participants will complete a 30-min test battery every 2 h to assess cognitive function and self-perceived capacity. Driving will be assessed before and after night shifts with a 20-min ‘commute’ in a driving simulator. • Days 3–16. Participants will have an 8-h sleep during 12-h breaks between night shifts. • Participants’ compliance with the protocol was monitored by research staff either in person or via a closed-circuit television system. All sleep periods will be monitored using polysomnography (PSG). Prior to sleep, a montage of PSG electrodes will be attached to the head and face. PSG data will be assessed in 30-second epochs using standard scoring criteria. The 30-min test battery will include tasks such as the Addition/Subtraction Task, Digit Symbol Substitution Test, Probed Recall Memory Test, Psychomotor Vigilance Task, etc. Subjective capacity will be assessed using visual analogue scales (e.g., fatigue, sleepiness, mood, alertness, etc). Body clock time will be assessed using the timing of the daily minimum in core body temperature (CBTmin), sampled in 1-min epochs throughout the protocol using ingestible capsules. Body clock adaptation will be assessed by collecting urine samples during and after sleep periods and expressing the amount of 6-sulphatoxymelatonin excreted during each daytime sleep period as a percentage of the amount excreted during the night-time baseline sleep period. Over the week of night work, percentages that are closer to 100 will indicate a greater degree of adaptation in participants’ body clocks.
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ACTRN12623000690651