VÍA JUDICIAL

Short-term consequences of sleep loss include increased daytime sleepiness and the intrusion of sleep into waking, as the homeostatic drive for sleep becomes so great as to be irresistible. The measurement of daytime sleepiness is a common method of quantifying the effects of insufficient sleep on daily functioning (Durmer & Dinges, 2005).

Excessive daytime sleepiness describes a state of decreasing vigilance, from full alertness towards sleep. Sleepiness is represented by several mechanisms including: physiologic change in the drive to sleep (a struggle for the brain’s alerting systems to hold sleep at bay); behavioural change such as the inability to remain awake and maintain psychomotor or cognitive performance; and change in a person’s introspective assessment of their state of alertness (Hirshkowitz, Sarwar, & Sharafkhaneh, 2011). Excessive sleepiness can be a dangerous state, such as when driving and in other safety critical situations. Excessive sleepiness is also associated with poor physical health, such as sleep disordered breathing. While common in pregnancy, excessive sleepiness may not always be normal and it has been associated with adverse pregnancy outcomes including gestational diabetes (Bourjeily, Raker, Chalhoub, & Miller, 2012; Bourjeily et al., 2013).

Cognitive performance has repeatedly been found to become impaired in sleep restriction studies, with the extent of impairment accumulating in a dose-response fashion (Banks, Van Dongen, Maislin, & Dinges, 2010; Belenky et al., 2003; Van Dongen, Maislin, et al., 2003). When comparing total sleep loss with chronic sleep restriction Belenky et al. (2003) argued that recovery of function occurs rapidly after acute sleep loss. Findings of their study suggested that chronic sleep restriction leads to a reduction in performance, which then stabilises, possibly as an adaptive response. The authors suggest that the cost of this response, however, is that a return to baseline function takes much longer than one or two nights of good sleep.

In another study, healthy adults who had sleep opportunity restricted to either 6 hours or 4 hours per night for 14 consecutive nights showed significant neurobehavioural degradation compared to subjects whose sleep opportunity was held at 8 hours per night for the same period (Van Dongen, Maislin, et al., 2003). Alertness, working memory and cognitive throughput were all measured and performance declined in all three tasks (psychomotor vigilance, digit symbol substitution and serial addition/subtraction tasks). The degree of impairment reached the same level as is seen after one to two nights of acute (total) sleep deprivation, depending on the task. Moreover, while cognitive performance showed no evidence of adapting to cumulative sleep loss, and continued to deteriorate across the study period, subjective ratings of sleepiness more-or-less stabilised after two days so that participants reported feeling only slightly sleepy at the end of 14 days of 6 hour or 4 hour sleep restriction. The authors observed that as long as people get at least 4 hours sleep per day they do not have a subjective experience of sleepiness at the same levels as those experiencing total sleep deprivation. These changes improve once there is extended opportunity for sleep but impairment continues to persist, albeit at a lesser level, for at least several days after the restriction period (Belenky et al.,

2003). By running the study for 14 consecutive days and nights the authors gained strong

evidence for their hypothesis that humans do not adapt to chronic sleep reduction – a

contrast to Belenky’s findings published at the same time (Belenky et al., 2003). For the present then, the amount of time required for complete recovery from sleep deprivation remains unknown and under debate.

Similarly, in a review of the effects of chronic sleep restriction, Banks et al. (2007)

concluded that people underestimate the cognitive impact of sleep restriction and overestimate their readiness to perform cognitive and behavioural tasks, including driving a motor vehicle. Driving is often highlighted as a task at risk of performance deterioration with reduced sleep, partly because of its real world relevance, and partly because, as a task, it reflects numerous components affected negatively by sleep loss, whether on the road or in a simulator (Bonnet, 2005).

Van Dongen et al. (2003) also found that the average amount of sleep needed to prevent cumulative neurobehavioural deficits was 8.16 hours per day, with prolonged wakefulness—as opposed to sleepiness—being hypothesised as the cause for the neurobehavioural deficits. They predicted that wakefulness beyond 15.84 hours (±0.73) produces such effects. Finally, these authors found the performance of some individuals to be more adversely affected by sleep loss than others. Some people appear able to maintain adequate function with 5–6 hours sleep per night, where others may need 10–11 hours. This finding of a trait-like vulnerability to sleep loss has been supported in subsequent studies (Tucker, Dinges, & Van Dongen, 2007; Van Dongen et al., 2004).

To put these consequences into a more ecologically relevant context, a number of studies have compared the reductions in cognitive performance from sleep disturbance to those associated with alcohol consumption. Neurobehavioural performance is tested using a driving simulator, or batteries of tests measuring cognitive processing, accuracy, concentration, motor speed and coordination, in groups who have either ingested alcohol or been sleep deprived. These studies have found that, after one night of sleep deprivation, performance decreases so as to be akin to a level of impairment induced by a blood alcohol concentration (BAC) between 0.08% to 0.10% (Dawson & Reid, 1997; Lamond & Dawson, 1999; Williamson & Feyer, 2000). The legal BAC limit permissible to drive a motor vehicle in New Zealand, the United Kingdom and the United States of America is 0.08%. Further, while performance remains reasonably poor but stable under the influence of alcohol, in sleep deprived groups, performance continues to decline across the duration of the study (Hack, Choi, Vijayapalan, Davies, & Stradling, 2001).