Neurochemical studies have revealed that the hippocampus and prefrontal cortex, brain regions integral to the process of memory, are rich in receptors for hormones that are activated by stress and sleep deprivation and which have been shown to have deleterious effects on memory. "To briefly summarize a vast, complex literature, prolonged and extreme stress inhibits the biological processes believed to support memory in the brain," says O'Mara. "For example, studies of extreme stress with Special Forces Soldiers have found that recall of previously-learned information was impaired after stress occurred." Waterboarding in particular is an extreme stressor and has the potential to elicit widespread stress-induced changes in the brain.
This matches something that occurred to me when I first heard about sleep deprivation and torture-- if I'm even moderately tired, it's much harder for me to remember details about when, where, and with whom something happened.
Link thanks to ookpik.