Did the research distinguish the cause or merely guestimate the cause (“ego depletion”)–? And does Cause then filter through other factors such as ‘Who causes sleepiness’ prompting new assumption on causative factors? Holographic thinking needn’t be linear–‘Educated authority’ (that which comes of learning by taking tests) tells us sleepy people do this perhaps moreso –learn in the process of taking tests– which learning may be affected by conditions present…that of sleepiness–which is but a mental regime with its own ‘speed’ parameters that may have nothing to do with prejudice against scholastic, qualifications, but for example ethno-lingo-historic use of verbiage,-twisting. Would you hire an Applicant who uses the word “logical” to mean talk-a-lot?

Sleepiness probably drives the attention-word-set more than the span.