Sleep improves prospective remembering by facilitating spontaneous-associative retrieval processes

Abstract

Memories are of the past but for the future, enabling individuals to implement intended plans and actions at the appropriate time. Prospective memory is the specific ability to remember and execute an intended behavior at some designated point in the future. Although sleep is well-known to benefit the consolidation of memories for past events, its role for prospective memory is still not well understood. Here, we show that sleep as compared to wakefulness after prospective memory instruction enhanced the successful execution of prospective memories two days later. We further show that sleep benefited both components of prospective memory, i.e. to remember that something has to be done (prospective component) and to remember what has to be done (retrospective component). Finally, sleep enhanced prospective remembering particularly when attentional resources were reduced during task execution, suggesting that subjects after sleep were able to recruit additional spontaneous-associative retrieval processes to remember intentions successfully. Our findings indicate that sleep supports the maintenance of prospective memory over time by strengthening intentional memory representations, thus favoring the spontaneous retrieval of the intended action at the appropriate time.

Figure 1. Prospective memory task and experimental design.

(A) During learning, subjects practiced on the lexical decision task first, which required them to press one of two buttons indicating whether the presented word was a real word or not. Thereafter, subjects learned 20 cue–associate word pairs. For instruction of prospective memory, subjects were then told that at retesting on the lexical decision task two days later, some of the 20 cue words could occur within this task and if they detected a cue word they should press the 'space' bar and type in the respective associated word. Subjects were explicitly told that they need to memorize this instruction because the experimenter would not remind them of what to do at retesting. In order to manipulate available attentional resources, subjects performed at retesting in parallel a secondary task (monitoring spoken digits for two consecutive even digits) either during the first or second half of the lexical decision task. (B) Experimental design: Learning (L) took place in the evening (~22.00 h) followed by a night of sleep (sleep group) or wakefulness (wake group). Retrieval (R) was tested after another night of (recovery) sleep in both groups.

Figure 2. Prospective memory after sleep and wakefulness.

(A) A significantly greater percentage of subjects who slept in the night after prospective memory instruction (sleep group) compared to subjects who stayed awake (wake group) retrieved the intention at a retest two days later, i.e., detected at least one cue in the lexical decision task. (B) The percentage of cue-words detected (a measure of the prospective component) differed in sleep and wake groups depending on whether subjects paid full attention to the task or engaged in parallel in a secondary (auditory attention) task. Subjects who slept after instruction of prospective memory were completely unaffected by divided attention, whereas wake subjects were strongly impaired in cue detection specifically during divided attention conditions. (C) The percentage of recalled word associates relative to the number of cues detected (measuring the retrospective component) was higher in sleep compared to wake subjects. Means ± SEM are shown. * p < 0.05, *** p < 0.001.