Diurnal Performance Decline in Narcolepsy: Limitation of Processing Resources

Vigilance decline in narcolepsy has been observed repeatedly in task performance . However, the nature of this vigilance effect is not exactly clear. In order to investigate vigilance problems in narcolepsy, it would be informative to examine to what extent narcolepsy patients differ from healthy people in naturally occurring vigilance fluctuations throughout the day. A general tendency to perform worse has been reported , and this suggests that vigilance is constantly at a lower level. On the other hand, a greater post-lunch dip has been found on some measures . It has been argued that the most parsimonious explanation is an impairment of the circadian process in narcolepsy . Conversely, it is argued that an increased homeostatic sleep drive may affect the diurnal pattern. In most studies on narcolepsy, patients are allowed to nap 3, . Naps are known to increase performance and alertness. The alerting effect of a nap may well influence results and might cause a relative improvement in performance in the afternoon. Therefore, vigilance may decline continuously over the day in narcoleptic patients if naps are not permitted. In this case, the homeostatic sleep drive is likely to be increased, a view that has been suggested before . Although a relation between vigilance and memory is plausible, the literature does not fully clarify this issue, although it is agreed that a short nap seems to improve performance in narcoleptic patients . In view of the experimental results mentioned above, three hypotheses are tested. First, it is predicted that an enhanced sleep drive can explain the pattern in diurnal variation for narcoleptic patients. Hence, it is expected that performance and alertness in patients will gradually decline during the day if naps are not allowed. Secondly, it is predicted that narcoleptic patients are impaired on the demanding task in particular. Thirdly, it is hypothesized that vigilance and episodic memory performance are related. There should be a simultaneous drop in memory performance if vigilance declines and variation in memory performance should correlate with changes in vigilance.