No abstract

Sleep loss is common in our 24/7 society with many people routinely sleeping less than they need. Sleep debt is a term describing the difference between the amount of sleep needed, and the amount of sleep obtained. Sleep debt can accumulate over time, resulting in poor cognitive performance, increased sleepiness, poor mood, and a higher risk for accidents. Over the last 30 years, the sleep field has increasingly focused attention on recovery sleep and the ways we can recover from a sleep debt faster and more effectively. While there are still many unanswered questions and debate about the nature of recovery sleep, such as the exact components of sleep important for recovery of function, the amount of sleep needed to recover and the impacts of prior sleep history on recovery, recent research has revealed several important attributes about recovery sleep: 1) the dynamics of the recovery process is impacted by the type of sleep loss (acute versus chronic), 2) mood, sleepiness, and other aspects of cognitive performance recover at different rates, and 3) the recovery process is complex and dependent on the length of recovery sleep and the number of recovery opportunities available. This review will summarize the current state of the literature on recovery sleep, from specific studies of recovery sleep dynamics to napping, ‘banking’ sleep and shiftwork, and will suggest the next steps for research in this field.

The light-dark cycle is the primary time cue for the synchronization of the internal human circadian system with the external world. Our previous research has shown the capacity of the circadian system to adapt to seasonal variations in the light-dark cycle at latitude ~40oN in the Rocky Mountains following curtailment of artificial light exposure. In the current study, we examined the circadian response to a longer photoperiod in summer by measuring melatonin timing before and after a week of camping outdoors in Sweden. Actigraphy-derived (Actiwatch Spectrum) assessments of sleep, activity and light exposure were carried out for 2 weeks. Week 1 was under modern urban living conditions with exposure to natural and artificial light in Stockholm, Sweden (~59oN; 18 hr 37 min: 5 hr 23 min light-dark cycle). Week 2 was spent rough camping in nature with exposure to only natural light at the Swedish High Coast (~63oN; 20 hr 40 min: 3 hr 20 min). Dim-light melatonin onset (DLMO) assessments were conducted in laboratory over two 24 hr periods immediately before and after camping to assess melatonin levels in hourly saliva samples. On average, participants’ (n = 10) melatonin midpoint and offset advanced ~0.7 and ~1.3 hr (p < 0.01), respectively, with no change in onset (p = 0.45). As sleep duration, and timing were similar between environments (p > 10), melatonin offset occurred closer to sleep offset after camping. Consistent with prior findings, the end of biological night is timed closer to waketime in the natural environment.