Napping on Memory and Cognition

Introduction

Throughout contemporary society, napping has had its unique connotations. Many find the practice to be lazy, [1] and a whopping 47% of American adults deliberately avoid taking naps [2]. Yet, receiving an adequate amount of sleep is constantly advised due to its abundance in recuperative properties such as the maintenance of metabolism, reduced risk of heart disease, as well as the improvement of memory and attention [3]. Naps are considerably shorter in length than nighttime sleep, with durations in this review ranging from 10 to 120 minutes. Despite this, numerous studies have shown positive correlations between napping and similar health benefits provided by nighttime sleep. Interestingly enough, a 6.5 year long investigation done on individuals from Spain, where habitually napping (known as “Siesta”) is a cultural norm. It was found that those who engaged in a 30 minute siesta daily had a 33% lower risk of developing obesity [4]. Along with its advantages, there are several drawbacks that arise from taking naps, which could possibly be a reason why the most favorable conditions for a nap are varied, as well as for why there is minimal promotion of napping in workplaces worldwide [5]. This review discusses the nuances of napping, and emphasizes the effects that this practice has on cognition. Current research strongly reinforces the profound impacts on memory, attention, and other various cognitive attributes that a short snooze can produce, as well as the optimal conditions for it.

Sleep Stages

The transition from wakefulness to sleep is known as sleep latency [6]. Once sleep latency ends, the sleep cycle begins. The sleep cycle is a process which consists of two consecutive phases: Non rapid eye movement (NREM) and rapid eye movement (REM). NREM is segmented into three consecutive subphases. The first stage is N1 sleep, where the eyes close, heart rate gradually slows, and one may experience sudden muscle jolts known as hypnic jerks [7]. This stage comprises approximately 5% of all sleep. The next phase is N2 sleep, where short bursts of neural firing, known as spindle fibers and K complexes, frequently occur [8]. About 50% of the time in sleep is spent in this phase. N1 and N2 are both considered “light sleep” [7]. The last stage of NREM is N3, alternatively known as slow wave sleep (SWS). This stage primarily consists of oscillatory brain activity known as delta waves, which have low frequency and high amplitude, also known as slow wave activity (SWA) [8]. SWA is taught to be the essential component of memory encoding and consolidation during sleep [9], which could be the reason why most studies discussing sleep’s effects on cognition focus mainly on N3 sleep. Waking up while undergoing SWA is said to result in sleep inertia, which Cassie J Hilditch and Andrew W McHill define as “the grogginess”, disorientation, and temporary impairment in cognitive performance upon awakening, which declines as time spent in wakefulness increases. [10] Sleep inertia generally lasts anywhere from 30 to 60 minutes [11]. Further detail on this phenomenon is discussed in the “Drawbacks: Sleep Inertia” section. The final process of the sleep cycle is known as REM, namely for the fast paced eye movement that occurs. Additionally, dreams and high levels of brain metabolism occur. The REM stage comprises 25% of total sleep, and usually inaugurates 90 minutes into sleep [7] [8]. A typical bedtime sleep duration is subjected to 4 - 5 full sleep cycles in order to maximize its effect on the brain [8]. Naps do not contain as many sleep cycles as nighttime sleep; Sometimes, they do not comprise even one full cycle [12]. Yet, several reports have attributed napping with similar effects on memory and attention brought about by nighttime sleep. These findings suggest that even brief engagements with certain sleep stages present cognitive benefits, or that some stages have a disproportionate effect on the brain than others. This raises the prominent question: which sleep stages and other qualities must a nap have in order to maximize cognitive processes, such as memory and attention?

Napping’s Effect on Memory

Throughout various settings, napping has demonstrated enhancements in memory encoding and consolidation ability. NREM sleep is said to function in the consolidation of explicit, or declarative memories [12]. One testament to this statement is an experiment that tested students’ retention of learned content. A group of students took a 35 minute nap, while another group of students had downtime in the classroom, and afterwards learned a biology lesson. After being tested on the material, it was concluded that the nap group scored a higher average percentage of correct answers than the group which did not nap [13]. Another study by Leong et al. examined the effect of napping prior to photo encoding, and during retrieval testing, found that the 30 minute nap group remembered the greatest amount of photos [14]. One study tested the additional strategy of cramming before being tested on declarative memory, alongside a nap, and a relaxing break. Cramming initially demonstrated a similar level of memory retention as napping, however, after being tested on memory again one week later, only the nap group was able to retain most of their learning [15]. This being said, napping before an encoding task is effective for retaining the information. However, it was found that napping in the middle of a learning session has an alike effect. A similar study which utilized a 90 minute nap between two encoding periods concluded that napping increased the retention of the encoded information by an average of 21%, compared to not napping [16]. Furthermore, napping is shown to benefit the encoding of motor memory. Participants learned a finger tapping sequence, and afterwards, half took a nap, and the other half rested. The nap group was split: half the subjects took a nap which spent just 10 minutes in N2 sleep, and the other half took a 60 - 90 minute nap which included both SWS and REM, effectively isolating the cognitive impacts brought by just N2 sleep. Upon retrieval of the sequences, both nap groups’ performance improved compared to the rest groups. However, there was no significant difference in recollection between the group which underwent just N2 sleep, and the SWS + REM group, revealing that a short nap which just includes N2 sleep still produces meaningful improvements in motor memory consolidation [17].

Sleep Mechanisms That Facilitate Memory

The exact mechanisms which enable memory encoding and consolidation during a nap are still being researched. Even so, there is substantial evidence which supports napping’s enhancements on memory, particularly the SWA of the N3 stage [7] [9]. For example, a higher proportion of SWS in a single nap demonstrated greater delayed recollection of word pairs [18]. Furthermore, experiments that have imposed a stimulation of SWA during a nap have found a positive causal relationship between the oscillatory activity that characterizes SWA and increased memory consolidation [9] [19]. One widely accepted explanation of the consistent correlations between SWS and memory consolidation is the synaptic homeostasis hypothesis (SHY), created by Giulio Tononi and Chiara Cirelli. The theory states that the biological purpose of sleep is to restore synaptic homeostasis. Specifically, the synaptic potentiation which occurs during wakefulness consumes high amounts of energy [20]. In order to return to homeostatic energy levels, SHY proposes a process known as synaptic downscaling. During SWS, neural pathways undergo a widespread reduction in synaptic strength. Downscaling results in lower energy demands from the brain, as compared to wakefulness. Though, the energy reduction is selective; synapses which are recently formed, frequently activated through repetition, or are of higher relevance due to strong connectivity with previously formed neural pathways, are all downscaled to a lesser extent than synapses which have formed longer ago or have a weaker connection to previously formed pathways. This process increases the total synaptic signal to noise ratio by effectively strengthening the signals deemed as ‘important’ - such as newly acquired information - and reducing the irrelevant signals [21] [22] referred to as “noise” in signal processing terms [23]. Through its refining properties, synaptic downscaling during N3 explains how memory consolidation is brought about by sleep [21] [22], including naps.

More reasoning for the association of SWS with memory is that its delta oscillations are closely correlated with activity from hippocampus [18] [24], a compartment in the brain which plays a fundamental role in memory processes [25]. The hippocampus however does not just exhibit oscillations, but also the sleep spindles found in the N2 phase of NREM [26], which could be the reason why a higher concentration of sleep spindles from N2 have been correlated with a higher information retention rate [15] [27]. Interestingly, a greater proportion of time spent in N1 was also correlated with improved information retention [13].

While the vast majority of research done regarding sleep’s effect on cognition attributes the benefits to NREM sleep, REM sleep has been said to facilitate the consolidation of implicit, or nondeclarative memories [28]. These are defined as the unconscious recall of information: primed behaviors, classical conditioning, procedural and emotional memories [12]. Studies have found a positive correlation between relative time spent in the REM stage and implicit task performance [29] [30]. Suppression of REM sleep was shown to impair procedural memory, according to a review by Smith [31]. Though, the relationship between REM sleep and memory consolidation is not as pronounced as that with NREM sleep, and the current hypothesis which attempts to explain the relation is often found to be implausible or contradictory at times [32]. In support of that, the facilitation of even nondeclarative motor memories were positively correlated with N1 and N2, rather than REM [17].

Napping’s Effects on Attention, Alertness, and Brain Health

Beyond its role in memory encoding and consolidation, napping has been documented to enhance general brain health, as well as alertness and provide greater sustained attention. For example, a positive causal relationship was found between habitual daytime napping and total brain volume [33]. In support of this finding, the habit of napping for either <30 minutes or 30 - 90 minutes was shown to be more protective against mild cognitive decline, than the habit of not napping [34]. Moreover, napping has exhibited activation in the prefrontal cortex [35], a region in the brain that plays a crucial role in regulating attention [36]. Thus, studies have also been able to conclude that taking a nap can substantially improve the upkeep of alertness and attention [37]. A group of individuals who napped for one hour displayed greater sustained attention throughout various cognitive battery tests compared to those who did not nap [38]. By utilizing a 40 minute nap during a flight, pilots were able to maintain a consistent performance level throughout the entire operation. Pilots who did not partake in a mid-flight 40 minute nap’s performance deteriorated over time, highlighting napping’s effectiveness as an alertness management strategy [39]. The increase in alertness and attention is related to a decrease in homeostatic sleep propensity (HSP) that one experiences after a nap [40] [41]. In short, HSP is the readiness to fall asleep. HSP accumulates throughout wakefulness, and is at its peak before falling asleep. HSP increases as a result of an extended period of wakefulness, such as sleep deprivation. This can hinder the function of the prefrontal cortex [42], which has further adverse effects on tasks that require high amounts of alertness and sustained attention [5]. After a short nap, HSP is at its lowest [43], signifying improved alertness and attentional control [44].

Drawbacks to Napping: Sleep Inertia

While napping has its benefits on cognition, there are drawbacks. A confounding factor which has shown to adversely affect memory and alertness immediately after waking is sleep inertia [14][46 - 47]. Sleep inertia is defined as the “temporary disorientation and decline in performance and/or mood”, which usually occurs after longer nap durations [47] and/or waking up from SWS [8][48]. The duration of sleep inertia typically ranges from 30 to 60 minutes [47], which is why some experiments required the participants to take shorter naps, or engage in a period of wakeful downtime after their nap in order to minimize the effects of sleep inertia [13][15][18][46]. An additional factor which has demonstrated influence on sleep inertia duration is the circadian phase in which a nap occurs [39]. Sleep inertia is greatest during nighttime [48], as that is when the body undergoes several biological mechanisms to fall asleep, such as the secretion of melatonin [49]. In support of this, one study found that napping later in the night showed the greatest hindrance in vigilance [46], a key characteristic of sleep inertia. Given these circadian influences on sleep inertia, it is vital to examine other ways that cognitive functioning can be affected by a nap.

Drawbacks to Napping: Circadian Rhythm

The biological drive to sleep has interactions with your external environment in order for the body to successfully fall asleep, as well as maximize the benefits that snoozing has on cognition. This relationship can be best visualized by Alexander Borbely’s two process sleep regulation model [50].

In the model, HSP is an outcome of two mechanisms, Process S and Process C. Process S displays the biological need for sleep, which accumulates as one stays awake, reaches an apex just before sleeping, and declines throughout sleep. This process is extremely similar to, and often used interchangeably with HSP. Process C enacts circadian rhythm [51], which is the 24 hour biological clock which regulates the promotion of wakefulness and sleepiness [49]. The peaks in process C during wake stages represent high circadian arousal, which is the release of wake-promoting factors [51], such as histamine [52], while the depressions in the function symbolize the release of sleep-promoting factors. When Process S is high, and Process C is low, sleep occurs [51]. A revised version of Borbely’s model includes sleep inertia with the two processes [48].

Hitherto, napping’s mitigating effect on HSP has been described in a positive light. However, the reduction of HSP can be damaging in the context of HSP’s interactions with the biological circadian rhythm. If one were to take a nap at a time of high circadian arousal, such as two to three hours after waking [53], or the early evening (also known as the wake maintenance zone) [54], they may wake up at a time of low circadian arousal. Napping at a time later than 6 pm or longer than two hours has been linked with reduced quality nocturnal sleep [55], likely due to the lack of HSP at a time of high circadian arousal. This mistiming of the two processes is known as circadian misalignment [56], which is associated with several impairments in cognition, such as various circadian rhythm disorders, insomnia [59 - 60] and excess sleepiness during daytime [58], as well as impairments in cardiovascular and metabolic health [57][59].

Optimal Duration, Timing and Frequency of a Nap

Throughout this review, naps have demonstrated benefits for brain health, attention, along with both declarative and nondeclarative memory. Yet, certain nap durations and timings have also produced harmful outcomes, such as sleep inertia and circadian misalignment. By examining current research, this section determines the optimal nap length, timing, and frequency for cognition. The ideal nap would maximize its benefits while maintaining circadian rhythm and producing minimal sleep inertia, if any at all. Several studies observed that naps that span 10 minutes [14][45] are less likely to later cause sleep inertia, possibly because naps of such length typically do not enter SWS [48]. They are less likely to contribute to circadian misalignment as well. However, shorter naps have generally shown little effect on attention and memory consolidation [14][45], though these findings are not consistent across studies [60][61]. In contrast, naps of longer length did indeed bring sleep inertia, but the benefits on attention [62] and memory [14] were more pronounced compared to those for shorter nap lengths. Additionally, the experienced grogginess from sleep inertia typically subsides within a short time frame [47][63], and thus the cognitive benefits only remain. These findings overall indicate that naps of greater duration offer more robust cognitive benefits. It is important to note that an experiment which suppressed REM sleep on the participants observed a minimal decline in learned memory [64]. This reveals that contrary to previous belief, REM may be less essential for the consolidation of declarative memories, therefore a nap duration under 90 minutes is favorable for many, especially in the context of avoiding sleep inertia.

Beyond duration , the time of day that the nap is taken greatly affects the nature of sleep inertia - alongside circadian rhythm. Waking up from a nap taken later at night showed increased signs of sleep inertia, such as impaired vigilance [46], compared to a daytime nap [65]. Sleep inertia tends to be more pronounced in states of high HSP [66], such as night time. Sleep inertia may also manifest after a nap taken by sleep deprived individuals; One study where subjects stayed awake for 27 hours observed sleep inertia after just a 10 minute nap [67]. That being stated, the optimal time to take a nap would be during daylight, at a time of low circadian arousal, and early enough in the day to not interfere with nighttime sleep quality. It is said that humans have a mid day trough in circadian arousal, also known as the “afternoon dip in wakefulness” [68]. This phenomena is characterized by the increase in HSP during the afternoon [69], particularly from 1 - 4 PM [70]. Taking a nap in this time window is ideal to counteract the decrease in performance, as well as not contribute to circadian misalignment.

It is hard to pinpoint how often one should take naps due to the limited research, and its difficulty to obtain results without the influence of confounding variables. However, one study found that those who took one to two 90 minute naps in one week spent a greater percent of their nap in SWS, and experienced lower levels of daytime sleepiness in comparison to those who took three to four 90 minute naps in one week [71]. While there are cognitive advantages to habitual napping, it appears that an excess frequency leads to complications. For example, longer and more frequent naps is linked with a greater risk of Alzheimer’s disease, though causation is unclear [72]. One survey revealed that adolescents who napped twice or more in one week often went to bed and woke up later, as well as acquired lower amounts of nighttime sleep compared to those who did not habitually nap [73]. Although findings are scarce, the claim can be made that if one decides to take longer naps as a habit, the frequency should be no more than two per week. However, this number can vary depending on how influential the nap is on one’s circadian rhythm.

Ultimately, the optimal conditions for a nap is dependent on the desired outcomes. With the goal of enhancing cognitive processes, such as memory and attention, the ideal nap would be under 90 minutes, taken in the afternoon, and occurs no more than twice a week.

Limitations

The lack of standardization throughout the studies prevents the ability to generalize the findings to a wider array of individuals. For example, the amount of initial nighttime sleep that participants received was not stated for every study. Some studies actively deprived their participants [35] [41] [67] of a recommended nighttime sleep length of 7 to 9 hours [74]. This means that taking a nap could have been less effective for cognition than if an adequate nighttime sleep duration was obtained [75]. If an initial amount of nocturnal sleep was a standard value across all nap related studies, that would allow for it to be disregarded as a confounding variable affecting any results.

Moreover, there is no standard definition of a “habitual napper”. Many studies did not explicitly state the nap frequency which constitutes their definition of a habitual napper, and/or had the participants self-report if they were a frequent napper or not [33] [34]. Because of this, it may be precarious to make a generalized claim regarding habitual napping’s effects on cognition.

Conclusion

Napping, often connoted as lazy or unfavorable, has shown itself to be extremely beneficial for cognition - especially in the context of memory consolidation, the sustenance of attention, and more in ways similar to those of nocturnal sleep. Both REM and all stages of NREM appeared to have an effect on the consolidation of memory. Though in the context of declarative (explicit) memories, the SWA of the N3 stage along with the neural bursts of N2 showed the greatest influence. Cirelli and Tonini’s Synaptic Homeostasis Hypothesis was created to possibly explain how SWA exactly facilitates memory consolidation. Research done on REM sleep’s effect on memory consolidation is not as thorough, but current studies suggest it has a greater impact on nondeclarative (implicit) memory consolidation. Other cognitive benefits brought by napping include enhancing physiological brain health, providing a greater sense of attention and alertness through the prefrontal cortex, and reducing HSP. However, the reduction of HSP can be faulty due to the nature of circadian rhythm, and its likelihood to contribute to circadian misalignment. Another probable disadvantage is the post-nap feeling of grogginess known as sleep inertia, though it is nonpermanent. In order to keep the occurrence of these at a minimum, it is generally advised to take a nap less than 90 minutes. The nap must be taken at a time of high sleep propensity and low circadian arousal, such as the afternoon. While findings regarding nap frequency are low and heavily varied, it is recommended to take a nap up to two times per week. A clearer understanding of nap frequency can help increase and validate the breadth of its effects, and ultimately improve the current rhetoric on napping. In a society that often advocates for receiving an adequate amount of sleep due to its benefits, napping should be further promoted for the same reason.

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