Sleep and Prospective Memory Retrieval
Remembering What You Need to Do (When You Need to Do It)
This past July 4th holiday left me feeling tired. Like most people in the US, I heard fireworks at all hours of the night and early morning. July 4th can be a fun holiday, but the noise, the unpredictable, loud, startling noise, is a major sleep thief. There are ways to mask noise (with sound machines, ear plugs, ear plug-sound machine hybrids, etc.), but it’s not exactly safe for us to be completely unaware of our environments. For example, we don’t want to miss hearing a fire alarm because we block the noise of fireworks so well that we can’t tell if something near us catches fire.
July 5th was a difficult day for me. I forgot to do simple things, like grabbing my wallet before leaving my place. Then, I forgot to take my bike helmet with me after locking my bike. Let’s discuss how sleep (or the lack thereof) may impact these simple memory tasks and possible methods of avoiding memory failures.
What is Prospective Memory?
Prospective memory is remembering to do what you intend to do. Simply, it is executing your intentions.
To remember what we need to do at a later time, we must first remember our intention. Retrospectively, we retrieve the intention from our long-term memory. For example, in the evening, I might plan to go for a run the next morning. I have to remember I had the intention to run. Secondly, we must remember the intention at the correct moment. This is the prospective component. The memory may be based on a time-based cue (e.g., I want to run at 5:30 AM) or an event-based cue (e.g., I want to run at twilight). So, we have two components to remember, (1) our previous, retrospective intention and (2) the moment in which we want to execute the intention.
How Might Healthy Sleep Support Prospective Memory?
Healthy sleep prepares us to learn new information and effectively retrieve it later on. Sleep may be particularly helpful for tasks that require high levels of cognitive control, such as associative memory retrieval. This is when we bind information. For successful prospective memory, we must bind prior intentions with future cues.
We may remember to complete a prospective intention in a couple of ways. One is with strategic monitoring. If we want to pick up a gift for a friend while we’re in a certain part of town, we need to remember that intention after getting to that space. With strategic monitoring, we might maintain the intention in mind, intermittently reminding ourselves, “pick up a gift for Jane,” “don’t forget about Jane’s gift,” “Jane’s gift: get it,” etc. Alternatively, we might not think of it much at all. When we arrive in the area, we spontaneously retrieve our intention and buy Jane’s gift. Sleep may be especially beneficial for spontaneous memory retrieval.
A meta-analysis including 24 participant samples demonstrated sleep’s benefit on prospective memory. On average, sleep had a modest effect on helping people remember to execute later tasks. Memory that requires spontaneous retrieval may be most strongly associated with sleep. For example, in one study 121 college student volunteers were assigned to groups for different experimental protocols. In each of the four groups, the delay between the prospective memory instructions (intention) and subsequent task (execution) was varied. While performing basic categorization tasks, such as indicating whether words were in non-living (basketball) or living (dog) categories, they were instructed to complete other tasks. Specifically, the participants were asked to press the letter “Q” on the keyboard whenever they saw a specific word appear during their other tasks (e.g., press “Q” whenever the word “vase” is shown.). The volunteers in the sleep delay condition showed interesting findings.
Those who encoded the prospective intention before a 12-hour interval with sleep had better prospective memory than those who encoded the intention before a 12-hour interval of wake. Those in the sleep interval condition underwent encoding at 9 PM, and those in the wake condition at 9 AM. Interestingly, sleep preserved prospective memory intentions. The researchers demonstrated this preservation by comparing prospective memory performance in the sleep condition to that of just a few minutes. While those in the sleep condition showed no major differences, those who spent the day awake after encoding demonstrated poorer memory than the sleep condition and those in the short delay conditions (morning or evening). The researchers concluded to sleep on intentions rather than setting them at the beginning of the day. This same researcher found writing a to-do list in the evening might even help with getting to sleep more quickly.
While the research on relationships between sleep and prospective memory is interesting, it is not without limitations. Most studies are based on college students, and little is known about the impact of age-related changes in brain structure and function on prospective memory retrieval. Also, few studies have investigated detailed, objective metrics that may explicate how sleep benefits prospective memory. The authors of a meta-analysis on sleep and prospective memory suggest brain imaging studies for future research. There is much more to learn about these relationships and the underlying mechanisms that drive them.
How to Avoid Prospective Memory Failures
One of the simplest ways to avoid prospective memory failures is to use external aids. We should not solely rely on memory to remember. Instead, use tools. Keeping digital and physical notebooks may help us remember to do what we intend to do. In the study referenced above, the researchers required participants to write down instructions by hand to ensure encoding of the prospective memory task. Perhaps the physical act of writing an intention helps us encode it, consolidate it during sleep, and execute our intentions, spontaneously, at just the right moment.
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