Let's say you want to make sure to refill a prescription before the pharmacy closes, or give your dog her flea-and-tick meds in a few hours, or walk your "I'll-take-steak-and-no-plus-one" RSVP to the mailbox before the day's last pickup. You could set an alarm or reminder on your iphone. Or, you might revisit the days of yesteryear and tape an alarmist note to your front door: "RSVP or no dinner and chicken dance for you!"
Indeed, we have plenty of ways to remind ourselves to get shit done. Still, who among us hasn't rejected the notion of needing that nudge — "Timer? I don't need no stinkin' timer" — only to climb into bed and realize the day's must-do task remains undone. Remembering to remember to take care of tasks is called time-based prospective memory (TBPM). For plenty of people, remembering to follow through is tricky in any mind-state. But, recent research on TBPM suggests that sleep loss compounds the challenge.
Time-based prospective memory involves establishing that you need to do something at a later point (let's say in one hour), continuing to do whatever else you're doing in the meantime, and then interrupting your ongoing activity when the hour is up, without any person or machine telling you it's go-time.
The role of sleep in memory is well-established. Slow-wave sleep (aka deep sleep) facilitates memory consolidation, the act of converting new experiences and information into long-term memories. And there's increasing evidence for the role of REM sleep, the phase when most vivid dreaming takes place, in strengthening emotional memories. Typically, studies link sufficient, high-quality Zzzs with a capacity to recall facts, figures, faces and feelings. Sleep loss, on the other hand, corresponds to an increase both in forgetting what happened and inventing what didn't, i.e. creating false memories. Medical case studies on worn-down physicians even propose amnesia as a side effect of sleep deprivation.
But prospective memory isn't a feat of pure recollection. Instead, it draws heavily on executive function, the umbrella term for mental skills involved in cognitive control, meaning the process of willing oneself to complete a goal-oriented task that won't get done on auto-pilot alone. (E.g. you're not writing that performance review unless you make yourself do it.) Executive-function skills include focusing, task-switching, planning, inhibition control and working memory, which requires keeping one fact or idea on-deck while simultaneously processing other information. Sleep-deprived people appear to struggle with certain executive functions more than others, and scientists have floated various hypotheses to explain the connection between sleep loss and executive-challenged minds.
One 2015 study on TBPM supports what's known as the "vigilance hypothesis," which blames impaired smarts during sleep deprivation on reduced vigilance and arousal. In studies, sleep-starved people vary considerably in their ability to perform cognitive tasks. (Some people can pull all-nighters and still destroy the Sunday crossword puzzle; others can't get through a simple Monday clue.) But, sleep deprivation consistently corresponds to slowed-down reaction times on tests of attention and vigilance.
The study, published in the journal Sleep in 2015, involved 50 young-adult participants. Half stayed up for 24 hours while the other half slept normally. At 8pm (after the sleep-deprivation group had been awake for a day), all participants had to take a psychomotor vigilance test, a timed assessment of reaction speed to visual cues, and cognitive-reasoning tests. During the cognitive-reasoning portion, participants had an additional assignment: to press "A" on a computer keyboard once they'd been working for 20 minutes. Stripped of their time-telling devices, participants could only check the time by hitting a specified key — this was the test of TBPM.
Both groups performed about the same in cognitive reasoning. But, the sleep-deprived group showed lower vigilance and less ability to hit "A" as instructed. Of the well-slept group, 80 percent pressed A successfully and 12 percent forgot. Among sleep-deprived participants, only 32 percent hit A on time, whereas 40 percent forgot.
Interestingly, researchers saw little difference in time-checking behavior between the two groups. So, all participants checked the clock roughly the same number of times. But, despite keeping tabs on time, members of the up-all-night group still didn't get around to pressing A. The gap between their time-monitoring and button-pressing abilities sparked a hypothesis: Sleep deprivation, researchers surmised, affects the brain mechanism that integrates time-related information and execution of an intended task. In other words, sleep-deprived people don't forget what they need to do, but they falter when it comes to actually getting it done.
The results, study authors wrote, supports the above-mentioned "vigilance hypothesis" because both groups of participants could handle other executive-function tasks (cognitive reasoning), but fell short on vigilance, thought to underlie TBPM. Now, back to that thing you promised yourself you'd do.