Brain Activity During Sleep: Rem Vs Non-Rem

Sleep is a complex and dynamic process that affects how we function in ways that scientists are only just beginning to understand. During sleep, the brain cycles through two different types: REM (rapid-eye movement) sleep and non-REM sleep.

During REM sleep, the eyes move rapidly behind closed eyelids, and brain waves are similar to those during wakefulness. Breath rate increases and the body becomes temporarily paralysed as we dream. In contrast, during non-REM sleep, the eyes don't move, brain waves are much slower, and the body maintains some muscle tone.

REM sleep is associated with dreaming, memory consolidation, emotional processing, and healthy brain development. It was previously believed to be the most important sleep phase for learning and memory, but newer data suggests that non-REM sleep is more important for these tasks, as well as being the more restful and restorative phase.

Brain activity during REM sleep

During REM sleep, the eyes move rapidly behind closed eyelids, breath rate increases, and the body becomes temporarily paralysed as we dream. The first cycle of REM sleep occurs about 60 to 90 minutes after falling asleep. As part of a full night's sleep, sleepers cycle through three stages of non-REM sleep, followed by one stage of REM sleep. Each cycle through all the sleep stages takes 90 to 120 minutes to complete. With each new cycle, sleepers spend increasing amounts of time in REM sleep, with most REM sleep taking place in the second half of the night.

REM sleep is important for dreaming, memory, emotional processing, and healthy brain development. Dreaming mostly takes place during REM sleep, with dreams in this stage usually being more vivid than non-REM sleep dreams. The amygdala, the part of the brain that processes emotions, activates during REM sleep. Memory consolidation also takes place during REM sleep, with the brain processing new learnings and motor skills from the day, committing some to memory, maintaining others, and deciding which ones to delete.

REM sleep is also associated with distinct global cortical dynamics. In mice, elevated activation in the occipital cortical regions (including the retrosplenial cortex and visual areas) became dominant during REM sleep.

Brain activity during non-REM sleep

During non-REM sleep, the brain is not dormant. Instead, it remains remarkably active, with brain activity decreasing in some areas and increasing in others.

Non-REM sleep is composed of four stages, with the first being the changeover from wakefulness to sleep. During this stage, the brain begins to produce slow brain waves, and the body relaxes as heart rate and breathing slow down.

In the second stage of non-REM sleep, the brain produces bursts of rapid, rhythmic brain wave activity. This stage usually lasts longer than the first and accounts for a significant portion of time spent asleep.

The third stage of non-REM sleep is deep sleep, during which the brain produces slow brain waves called delta waves. This is the most restful and restorative stage of sleep, and it is more difficult to wake someone during this stage.

The fourth stage of non-REM sleep is REM sleep, during which the brain becomes more active, and dreams occur.

During non-REM sleep, the brain alternates between being active and quiet, with brain activity decreasing in the brainstem, thalamus, and several cortical areas, including the medial prefrontal cortex. However, brain activity increases in specific subcortical and cortical areas involved in generating or modulating sleep oscillations.

The brain's default mode network (DMN), which includes the posterior cingulate cortex, medial prefrontal cortex, and inferior parietal lobules, is functionally uncoupled during deep non-REM sleep and recoupled during REM sleep.

During non-REM sleep, the brain also reorganises and catalogues memories and learned information, making it easier to access and use them.

The role of REM sleep in memory consolidation

The brain remains remarkably active during sleep, and sleep is important for a number of brain functions, including memory consolidation. Sleep is vital for optimum acquisition and performance of learned tasks, but a major role in memory consolidation is unproven.

Evidence for increased REM sleep duration with learning

The idea that REM sleep duration increases with learning is based on the hypothesis that increased learning will require increased memory consolidation and hence more REM sleep time. However, animal learning studies cannot always guarantee that the novelty of a new experimental situation will produce a substantial overall increase in learning. It is quite likely that stress associated with shock avoidance, used in many REM sleep-learning studies, will have a major impact on the animal. The assumption that levels of stress are not correlated with the nature of the learning task and with the animal’s success at the task is unproven and unexplored in most of these studies. This issue is particularly worrisome because it has been shown that moderate stress, in the absence of any imposed learning task, can produce a marked increase in REM sleep.

Some studies have shown that increases in REM sleep duration occurred at different times after the imposed learning task. In some experiments, the REM sleep increase occurred immediately after training, while in others, it appeared with some delay, or even not at all. In one avoidance task, REM sleep increases were seen from 1 to 4 hours, 9 to 12 hours, and 21 to 24 hours after training, but not at other times. In other studies, the "REM sleep window" was said to last more than 15 days. These REM sleep windows were said to depend on the type of task, strain of animal, and number of training trials per session.

A REM sleep enhancement phenomenon has also been sought in human studies. Some of these studies have used prism glasses that distort the visual world. Such glasses perform a 90-degree rotation or inversion of the subject’s view. Over a period of days, subjects learn to adjust to these glasses. However, an initial abstract in 1970 that concluded that such an experience produced an increase in REM sleep was contradicted by a more thorough study using a similar paradigm, which found no such increase. Further work by the authors of the original abstract confirmed the absence of an effect, as did three additional studies using a variety of visual distortions. Another study using a somewhat different spatial rearrangement did find a small effect, with REM sleep increasing from 19% to 22% of total sleep time. These same authors noted a small increase in REM sleep during language learning, a type of task that others have concluded does not require REM sleep.

Evidence for the expression of learning processes during REM sleep

Several investigators have sought evidence to support the hypothesis that memory consolidation is occurring during sleep. The replay of neuronal activity seen during prior learning episodes might be evidence for mnemonic processes. However, a replay of neuronal events in subsequent REM sleep epochs might not be part of consolidation. Indeed, such replay might be involved in genetically programmed neuronal development, or may have a role in the extinction of memory traces.

Recordings from the motor cortex analog of zebra finches detected neuronal activity patterns in sleep similar to those present during waking singing, suggesting that a genetic readout of species-specific bird-song may be taking place. In this study, the nature of the sleep state (REM versus non-REM) in which these patterns were present was not identified. The idea that REM sleep has a role in genetic programming of behavior during neuronal development is supported by the relatively high amounts of REM sleep in early life in mammals.

Two recent papers have studied unit activity in the hippocampus of rats during REM sleep in a search for evidence of mnemonic processes. The first studied the firing of groups of neurons in the hippocampus, a structure known to be important in memory consolidation. The cells recorded were selectively active during waking in relation to the physical location of the animal within its environment. The authors compared the activity of “place cells” active in familiar places of the environment with those of place cells active in newly exposed portions of the environment. They found that each of these two categories of cells had differing phase relations to the theta rhythm in waking as compared to REM sleep. These findings suggested to the authors that REM sleep was exerting mnemonic functions, perhaps by strengthening memory traces linked to recent experience while eroding traces linked to more remote memories.

Another study examined more extensive samples of activity in hippocampal cells in rats and compared discharge patterns during REM sleep to those during training on a circular track. By expanding and contracting the duration of the REM sleep samples and using a sliding template to identify matches, it was concluded that a replay of waking hippocampal activity occurred during REM sleep. However, this “replay” was found primarily in REM sleep episodes occurring immediately before the daily learning trials, not in those occurring in the hours immediately after learning. The authors interpreted this as reflecting a replay of training sessions that occurred 1 day earlier, although there is no persuasive evidence for this interpretation. It is unclear why this “replay” was not seen in REM sleep occurring subsequent to the behavioral episode. Furthermore, when these animals were exposed to a novel training task, no replay was detected in any subsequent REM sleep period. These data do not appear to support the consolidation hypothesis.

If waking events to be consolidated are replayed in sleep, one might expect not only a replay of unit activity patterns but also a reactivation of the correlated mental experience. We have access to such experiences in humans who are awakened from REM sleep. A few recent papers have examined dream reports in subjects undergoing an intensive pre-sleep learning experience. In one such paper, fewer than 10% of dream reports contained any reference to a task just learned, and many of the dreams that referred to the learned task occurred after consolidation had occurred, not before. Language immersion learning and visual field inversion produced “relatively few direct incorporations of the learning material” into reported dreams. A review of the literature found that few dreams are linked to recent experiences, including new experiences that are subsequently remembered. The dream reports that do incorporate experiences from the prior day or two are rarely a “replay” of events or learned tasks. Instead, they are more likely to be linked to the situation in which the learning occurred or the emotions correlated with the

The role of REM sleep in emotional processing

The brain is active during both REM and non-REM sleep. During REM sleep, the brain is in an activated state, but remains functionally isolated, operating with reflective awareness despite the relative absence of exteroceptive input. The amygdala, an almond-shaped structure involved in processing emotions, becomes increasingly active during REM sleep.

REM sleep is thought to play a role in emotional processing. It is associated with increased emotional reactivity to negative stimuli, and may facilitate emotional processing during subsequent nights, leading to reduced intrusive memories in the long term. However, the findings are mixed, and the conditions under which higher amounts of REM sleep lead to decreased or increased emotional responses are unclear.

The role of REM sleep in brain development

REM sleep is a distinct "third state" of consciousness, during which levels of brain activity are similar to wakefulness. During REM sleep, the brain is highly active, and the body is temporarily paralysed. This state is characterised by rapid eye movement, irregular breathing, and increased heart rate.

REM sleep is important for brain development, memory consolidation, emotional processing, and dreaming. Dreaming is thought to help process emotions, and is more vivid during REM sleep. Memory consolidation occurs during REM sleep, as the brain processes new learnings and decides which to commit to memory. Brain development is also facilitated by REM sleep, as newborns spend most of their sleep time in this state.

REM sleep is generated and maintained by the interaction of various neurotransmitter systems in the brainstem, forebrain, and hypothalamus. The core of the REM-generating circuit is the subcoeruleus nucleus (SubC), which is composed of REM-active neurons. These neurons are predominantly active during REM sleep and are thought to regulate REM sleep and its defining features.

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