Author(s): William R. McMahon 1,2, Suzanne Ftouni 1,2, Andrew J. K. Phillips 1,2, Caroline Beatty 1,2, Steven W. Lockley 1,2, Shanthakumar M. W. Rajaratnam 1,2, Paul Maruff 1,2,3,4, Sean P. A. Drummond 1, Clare Anderson 1,2,*
Numerous behavioral factors adversely impact sleep quality or quantity. Collectively referred to as poor sleep hygiene, these adverse behaviors include irregular sleep timing, exposure to light prior to bed, daytime napping, and consumption of stimulants. To minimize the impact of these factors, participants in sleep and circadian research studies are often required to adhere to a structured sleep schedule for 1-3 weeks before an in-laboratory protocol. During this time, they are further prohibited from daytime napping and the consumption of alcohol and caffeine. A structured sleep schedule will typically allow participants an 8h sleep opportunity between times that are either selected by the participant (e.g., [1,2]), assigned to them based on their normal schedule (e.g., ), or identical for all participants (e.g., ). These pre-laboratory procedures are intended to stabilize participants' circadian timing, satiate sleep need, and homogenize participants' sleep outcomes prior to interventions.
The timing and duration of sleep are regulated by circadian and sleep homeostatic factors . Sleep is more difficult to initiate, and of reduced duration and quality, when its timing is not optimally aligned with the circadian drive for sleep [6,7]. For instance, awakenings are more frequent and the total duration of sleep is reduced in sleep episodes initiated during the circadian day, when endogenous melatonin levels are low . Even relatively minor misalignment between the circadian clock and sleep/wake timing can result in poorer quality sleep, especially if sleep initiation occurs close to, or before, the evening secretion of melatonin, during the wake maintenance zone [9,10]. Moreover, light is the primary time cue that resets the circadian pacemaker , and the timing of light exposure relative to circadian phase determines whether it will advance or delay the circadian rhythm [12,13]. A more irregular sleep schedule pattern will lead to greater day-to-day changes in the timing and duration of light exposure and therefore greater instability in circadian timing on a day-to-day basis . Theoretically, structured sleep would provide a more stable circadian phase and provide for an adequate sleep opportunity aligned with their circadian timing, but circadian phase is rarely assessed pre-study.
Beyond optimizing circadian timing, the time in bed permitted by a pre-study structured sleep schedule attempts to minimize chronic sleep deficiency prior to a laboratory study. This is especially important for studies involving sleep deprivation, where existing sleep deficiency impairs tolerance of subsequent acute and chronic sleep loss [15,16]. Consensus statements from the American Academy of Sleep Medicine and Sleep Research Society, and the National Sleep Foundation, suggests that individuals should obtain 7-9 hours of sleep each night for optimal human health and performance [17,18]. Indeed, in studies of healthy young adults given extended sleep opportunities of 12-16h, sleep duration eventually reaches an asymptote of 8.2-8.7h [19-21], while modeling of daily neurobehavioral responses to varying sleep opportunities...