Beyond Rest: The Neurobiology of Naps and Their Impact on Cognitive and Emotional Health

Introduction

Sleep is a basic behavior of all animals and while it points to some inherent biological functions, what we don't quite understand is the neurobiology of short durations of sleep; what we refer to as naps. Current evidence is suggesting that even brief episodes of sleep contain several neural processes that span a range of brain regions, and they certainly impact cognitive function, emotional well-being,  and overall health. Research shows that simple periods of rest have the ability to change neural pathways/connections between neurons that allow communication throughout the nervous system, which impact performance upon waking. Recent developments in neuroscience are quickly elucidating the pathways that naps foster. They’re also explaining how naps may decrease the activity of key neurotransmitters that are tied to restorative brain processes. The ability to comprehend the complex neural processes involved in napping, fundamentally changes  our ideas about sleep during the day to a potential tool for optimizing cognitive and emotional health.

Purpose of Sleep and the Circadian Rhythm

Sleep serves the function of restoring homeostasis in the brain. [1]  While sleeping, the brain turns on the glymphatic system in the brain, which cleans up and takes away metabolic by-products made during wakeful states. [1] This process is very important, as the collection of these by-products can cause neurodegenerative diseases. For example, an excess of beta-amyloid proteins is strongly correlated to developing Alzheimer’s disease. Another function of sleep is to complete memory processing and increase neural plasticity, the brain’s ability to reorganize and create subsequent neuronal connections. Neuroscience supports sleep's role in  memory consolidation and other plastic neuronal changes. [1] Sleep allows the brain to restore, reorganize, and prepare for optimal performance in cognition the next day.

Circadian rhythm, a roughly 24-hour internal clock that dictates the sleep-wake cycle, is just as critical to cognitive functions as it is to sleeping and waking. This biological clock allows humans to anticipate and physiologically respond to regularly occurring daily environmental conditions via exogenous and endogenous mechanisms (Schwartz & Klerman, 2019). The biologically-based circadian system is governed by the suprachiasmatic nucleus (SCN), a cell cluster in the hypothalamus that dictates rhythms of rest and activity. Schwartz and Klerman, (2019) further suggest that the circadian rhythm is paramount to interval timing and duration of rest, activity, eating, and other biological functions for organisms in their ecological niche, and thereby adapt successfully to seasonal changes in the length of daylight. [2] In a human context, a healthy and functional circadian rhythm promotes consolidated sleep state at night and sustained levels of alertness when awake, that are critical for optimal cognitive function. Nonetheless, disturbance to the circadian rhythm (i.e., irregular or drastic rotating work  shifts and/or rapid travel across multiple time zones) misalign sleep-wake schedules, which can result in cognitive deficits, mood disturbances, and more systemic health consequences.

Sleep Stages and the Brain’s Sleep Circuit

Sleep consists of multiple, distinct stages that cycle repeatedly during the entire resting period. Sleep stages are classified as non-rapid eye movement (NREM) sleep with stages 1 to 3 of non-REM sleep and rapid eye movement (REM) sleep. Brain activity substantially slows down during NREM sleep, especially during the deep stages, called slow-wave sleep, the body is then relaxed and large synchronously occurring waves drift across the cortical surface and cerebral cortex. REM sleep is a different state where the body is more like in a state of  paralysis, the brain is active and dreaming, and brain waves become desynchronized. Both non-rapid eye movement and rapid eye movement sleep are important. Slow-wave NREM sleep allows the brain to process memories,especially facts, and clear metabolic waste products; REM sleep processes memories that are emotions and makes more diverse neural connections. Nap times usually just include the lighter NREM sleep stages (1&2) and possibly slow-wave (deeper) sleep stages. Brief periods of REM may be involved depending on the time remaining in the nap period. The restorative benefits of short naps are similar to a full night’s sleep, but on a smaller scale.

The changes between different sleep stages and wake that happen throughout the day and night are coordinated by a range of brain circuits and regions. The suprachiasmatic nucleus is a central part of this network commonly referred to as the master clock (Dijk & Cajochen, 2010). The SCN generates rhythmic signals that are sent to the hypothalamus to influence wake-sleep states. When neurons in the ventrolateral preoptic nucleus (VLPO) of the hypothalamus are activated, they reduce the brain’s arousal centers to promote sleep. [3] The brain’s arousal centers are mainly composed of regions in the brainstem and basal forebrain that stimulate wakefulness through neurotransmitters that promote arousal such as norepinephrine, acetylcholine, histamine, and orexin. VLPO neurons release gamma-aminobutyric acid (GABA), which is an inhibitory neurotransmitter that inhibits the wake-promoting regions of the brain by disrupting coordinated neuronal communication and thereby limiting alertness when going through sleep onset. The brain uses a second group of hypothalamic neurons that produce orexin (or  hypocretin) to amplify the mechanisms that promote stable transitions between wake and sleep. Orexin neurons help to optimize the amount of time awake and limit sudden shifts into a sleep state. [3] When orexin neurons are severely reduced or absent , as seen in the sleep disorder narcolepsy, the effective regulation of wake and sleep transitions is lost because of unexpected sleep episodes.However, ordinarily, the SCN, VLPO, and arousal networks all interact in an organized, complementary manner to modulate the timing, quality, and nature of sleep, including dreaming.

How Sleep Shapes the Brain

Sleep creates structural changes in the neural circuits of the brain. Research indicates that the brain is involved in sleep-dependent memory consolidation, an act which uses neural activity patterns that were previously developed in wakefulness. "Replay" is specifically emphasized in the scientific journals  referring to memory traces from the day that are reactivated, to further strengthen the synapses associated with any newly acquired information or newly developed skills. This neural replay is key to both the stabilization of the memory and its integration with previous knowledge. Sleep is also involved in synaptic pruning, which involves the removal of unwanted or redundant neuronal connections keeping neuronal networks as efficient as possible. Research shows that during sleep signals within synapses are examined for usefulness. [4] Connections that are not relevant or not used are weakened and may be lost while connections that are important or more used get strengthened and defined. These assessment and modifications of neural connections established by previous activity are essential for ensuring continual neuroplasticity. In fact, robust evidence establishes the importance of sleep for ensuring neuroplasticity across multiple stages of sleep.

In a meta-analytic review, Matthew Walker and Robert Stickgold (2006), determined that particularly the slow-wave and REM portions are critically involved in facilitating various types of learning, including the acquisition of factual information and motor skills. [4] They also concluded that a nap might be sufficient to achieve the same results. In fact, one classic study on a visual pursuit task compared the results of trained participants. They were split into two groups. In one group, the members took only a nap and in the other group, the participants slept overnight for 8 hours. Both groups showed significant improvement in the task. Based on the outcomes , the authors concluded that a nap was equally as sufficient as a night of sleep regarding the behavioral improvement for the perceptual task. Both of their studies support the suggestion that there are considerable and consistent benefits of sleeping. During rest, the continued alterations of the brain's performance and thought processes positively influence a person's ability to develop skills, gain insight, and easily find creative solutions to problems. [4]

The Benefits of Sleep and Napping

Sleeping and napping benefit the brain and cognitive functions in multiple ways. Essentially, both improve memory. During sleep, both declarative and procedural memories such as recalling facts or events and unconsciously performing skills, respectively, are consolidated. A nap helps transfer short term memory to long term memory. In other words, it helps with making recently learned information more permanent in the brain, specifically in the neocortex. Studies with young adults demonstrated that a nap taken in the middle of the day prevents the normal decrease in alertness as the day progresses and improves memory test scores following that nap. [5] An indication that the brain is working to solidify memories even during naps, is that brief bursts of brain waves during NREM called sleep spindles showed better memory consolidation.

Besides providing advantages for memory, sleep and naps are also beneficial for overall cognitive function. For example, a systematic review on naps of a short duration showed enhanced alertness, reaction time, executive function, and creative thinking (Dutheil et al., 2021). [6] Many people use a short afternoon "power nap" to help readjust their levels of concentration and therefore productivity later in the day.  A nap as short as 20 minutes can reduce homeostatic sleep pressure. Homeostatic sleep pressure, could be described as sleep deprivation, increases the longer someone is awake. and naps can restore alertness for a few hours. Naps are also associated with improvements in mood. People often report a decrease in  stress,  irritability, and being emotional after napping, presumably because the regions of the brain responsible for emotional regulation (e.g., the prefrontal cortex and limbic regions) had a chance to recover from the fatigue of being active for a long time, allowing the brain to return to a more rounded stable state. So, in summary, extended sleeping periods and short naps are both essential maintenance processes for the cognitive and emotional systems in the brain to increase our ability to learn, process information, make decisions and exercise creative problem-solving skills. Sleep provides these cognitive and emotional benefits in a pace related to the amount and quality of sleep you obtain. This underscores why even a short nap can effectively restore cognitive capability in your day.

Conclusion

In conclusion, sleep (which can include both lengthening sleep at night or napping during the day) is an important biological behavior that promotes core functions of health related to cognitive, emotional, and brain health. Sleep neurobiology occurs via many different levels of interaction across regions of the brain.  The complex interaction between coordinated circadian rhythms and the  suprachiasmatic nucleus not just activates  fundamental neuronal circuits but also exchanges information between sleep stages. These various forms of sleep all have purposes to promote memory consolidation, cognitively enhanced performance, emotional functions such as self-regulation, and other neuroplasticity related functions. While indeed small napping periods may not be as effective syntactically for specific neuroplastic changes, they certainly have a sufficient restorative function that reinforces cellular connectivity, funds neural responses, promotes affective mood states, and enhances subjective alertness. As contemporary neuroscientific literature exists, it hopes to disclose the precise neural link to various processes through sleep or napping. As a maintenance of how to view various periodic geometric aspects of sleep that are probably not merely a passive state, they are actually multi-faceted processes important to our cognitive and emotional health. Of course the implications of future research studies both to understand sleep or napping will only enhance our base of knowledge, but possibly more importantly could develop specific proposals that could influence our overall daily functioning and perhaps help mitigate neurological maladies.

References

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