The effect of a short nap following 4h sleep restriction on pain sensitivity in young healthy males

Abstract

Introduction: Sleep is a very important biological process and has been linked to memory cementing, recuperation and regeneration of cells and other bodily systems such as the immune system. Sleep disruption (restriction, deprivation and fragmentation) results in decreased cognitive function, higher reaction times, and increased risk of cardiometabolic dysfunction. A bidirectional relationship between sleep and pain perception has been well documented. Curtailed sleep results in higher pain sensitivity and lower pain thresholds; on the other hand people who are in pain (especially chronic pain syndromes) have been shown to sleep worse than those who are not. Thus, a lot of interest has emerged on the possible recuperative effects of recovery sleep, particularly in the form of a daytime/afternoon nap. There is, at the time of this study, only one other paper that has investigated afternoon naps in relation to pain perception. There is still conflicting evidence as to the length of nap that gives maximum physiological benefits and recuperation (pain, cognitive and immune function). This study aimed to investigate the effects of two kinds of sleep restriction, restricting the first four hours (SR-F4H) and the last four hours (SR-L4H) of sleep, on sleep architecture and pain perception in healthy young males. Furthermore, to grow the literature on naps and pain, I aimed to investigate the potentially recuperative effect of a short afternoon nap following the nights of sleep restriction as well as the relationship between nap sleep architecture and subsequent pain. Methods: 11 healthy young men (Median [IQR], 21 [20 - 21] years old) were recruited to participate in the study using flyers and word of mouth. They had healthy-slightly overweight Body Mass Index (BMI’s) (22.3 [20.7-25.3] kg.m2 ) and healthy 7-9 hour sleep/wake cycles. They were screened for a week before commencement of the study using the General Health Questionnaire, Pittsburgh Sleep Quality Index, Sleep Diaries and Actiwatches. They then entered the experimental portion of the study. They had one adaptation night followed by five condition nights (Baseline, SR-F4Hnap, SR-F4Hnonap, SR-L4Hnap and SR-L4Hnonap) that were in random order. The participants would also have afternoon procedures where they either came in for a 30-minute nap or they came into the laboratory for a 30-minute lie-in (where they lay down for 30 minutes in a dark room). Following the night and afternoon polysomnography, they underwent a 10 minute ischeamic pain test, induced using a tourniquet. The pain was scored on a 0-100mm visual analogue scale every minute until the 10 minute mark. I used mixed models in SAS 9.4 ® to investigate 1) the relationship between experimental conditions and sleep composition, 2) the relationship between pain perception and experimental conditions when adjusting for order of conditions, time of day, presence of a nap and time during the test 3) the relationship between pain perception and sleep stage composition during the experimental night preceding the pain tests, when adjusting for the same variables 4) the relationship between pain perception in the afternoon and sleep stage composition during the nap. Results: Total Sleep Time (TST), N2, REM (in minutes and proportions) were significantly decreased during sleep restriction as per the design of the study. Proportions of TST, N2 and REM showed a protective relationship with pain, whereby an increase in these variables resulted in a decrease in pain perception (p = 0.006, p = 0.003 and p = 0.007 respectively). On the other hand, N3 showed a positive association with pain, whereby increases in N3 resulted in increased pain scores (p = 0.006). The proportion of N2 was the most reliable predictor of pain the next day, where increases in N2 were directly associated with decreased pain perception upon pain testing (p = 0.003). Pain scores were lower in the afternoon compared to the morning (p<.0001). The nap condition was associated with increased pain perception (p<.0001). Pain perception decreased throughout the study (order effect p <.0001), potentially a result of habituation of the participants to the procedures. The minutes spent in N3 sleep and the longer time awake between waking in the morning and the afternoon nap were 3 significant predictors of pain, where longer times resulted in higher pain reported by the participants (p <.0001 for both). Conclusions: Sleep restriction, whether SR-F4H or SR-L4H, did not result in higher pain perception in these participants however pain perception was increased with higher N3 and lower N2, TST and REM. Furthermore, the 30-minute afternoon naps seemed to increase the pain perception, mainly through time spent in N3 sleep. The relationship between specific sleep stages and pain perception and the effect of recovery sleep still needs to be further elucidated