The K-D test is sensitive to the impact of sleep deprivation on cognitive functioning, including aspects such as rapid eye movements, concentration, and language. Participants not taking call showed a median improvement of 3.8 seconds in follow-up K-D scores, consistent with the learning effect previously described in K-D testing.7,8
However, residents had a median slowing of about 0.23 seconds on postcall K-D testing, suggesting that the learning effect was negated by sleep deprivation. While this worsening in K-D test performance is not as extreme as seen in a cohort of athletes with concussions (reported median 5.9-second increase in K-D time),8
it demonstrates the potential effectiveness of K-D testing to detect eye movement slowing in sleep deprivation. Furthermore, postcall residents had an increased number of errors on follow-up relative to control subjects, similar to the increased number of errors observed in athletes with concussions.8
These results suggest that increased time and error on K-D testing capture the deleterious effects of sleep deprivation on eye tracking and attention.
Less sleep in the prior 24 hours was associated, in the overall cohort of on-call and noncall participants, with less improvement in K-D time from baseline. However, among residents taking call, no association was found between duration of sleep obtained on-call and K-D follow-up performance. Nonetheless, the performance on the K-D test was impaired in this group. In other words, postcall residents obtained less sleep time than subjects not taking call, which correlated to poor performance on the K-D test, but when a subanalysis of the postcall residents was completed no correlation between K-D time from baseline and sleep was observed. The finding is unlikely to be due to variability in test performance; in fact, recent studies demonstrated a high degree of test-retest reliability for K-D with intraclass correlations of 0.97.7
The result could be explained, however, by interindividual vulnerability to the effects of sleep deprivation that has been documented with other sleep deprivation measures.1,3
In the current study, some residents obtaining zero hours of sleep on call improved at a level similar to controls while others were significantly impaired in their K-D performance even when obtaining several hours of sleep on call. No study variable (including age, sex, time to caffeine, level of training, hospital service, or prevalence of sleep deprivation symptoms) effectively predicted which individual would be more affected by sleep deprivation. One potential confounding factor may have been the amount of caffeine consumption prior to testing; while our study captured the time since last caffeine intake, the actual quantities of caffeine were not evaluated in this study.
Self-report of increased level of sleepiness (using the KSS) was associated with less improvement in K-D times from baseline. This finding corresponds to prior research that found participants reporting an increased level of sleepiness had slower peak eye saccade velocities relative to participants rating themselves as more alert.5
Although the ability to appropriately identify level of alertness has been debated, recent evidence suggests that subjective alertness and performance are modestly correlated, with most discrepancy during the biological night.10
The association between subjective level of sleepiness and improvement in K-D test times in the current study provides further support for the concept that individuals are fairly effective in rating their level of alertness. However, self-report of sleepiness is a subjective measure not capable of capturing individual vulnerability to sleep deprivation effects.
Severe fatigue (ratings sleepy and very sleepy on the KSS) significantly affected follow-up performance on the K-D test in the present study. Since prior research has demonstrated no effect on K-D performance from athletic workout fatigue,8
the current results extend our understanding of K-D testing in relation to extreme fatigue levels. In the search to objectively evaluate the effects of sleep deprivation, the K-D test offers a simple and quick method to measure degree of eye movement slowing in subjects. Further research with larger cohorts is needed to expand the subject sample size, elaborate interindividual variability in vulnerability to the effects of sleep deprivation, and evaluate the impact of different sleep loss patterns on attention and eye movements. Indeed, the K-D test offers the potential to monitor resident performance under a variety of call schedules, including night float systems, and to test the association between eye movement slowing and clinical errors.