It’s no surprise that a poor night’s sleep leads to “brain fog” while a good night’s sleep makes us feel alert and sharp all day long—but the effect of sleep deprivation extends far beyond feelings of next-day fuzziness. Below, we break down the complicated relationship between sleep and memory systems.

There are four different stages of sleep that the human body cycles through over the course of the night. The duration and purpose of each stage differ, as does the “type.” On the one hand, you have non-rapid eye movement sleep (NREM sleep or non-REM sleep), which is known as quiet sleep and has three phases. And on the other hand, rapid eye movement (REM sleep), which is more active.

Cognitive functions such as memory consolidation, creativity, and learning are fostered during REM sleep. During this stage, which we spend about 20-25% of our night in, brain waves are far more active, although muscles remain fully relaxed—which is a good thing! This is the prime time for dreams to occur, and we don’t want to be acting those out. Because of the disparity between the brain increasing activity and the muscles remaining inactive (known as atonia), REM is sometimes referred to as “paradoxical sleep.” (1-2)

Depending on the type of memory (more on that below), stage 3—also known as N3—may play a role. During this time of deep, restorative sleep called “delta sleep” or “slow-wave sleep” (SWS), it’s difficult to wake up. The brain is releasing low-frequency, high-amplitude delta waves that cause heart rates and respiration to slow down.

READ MORE: 6 Ways Sleep Deprivation Affects Work Performance

  1. Declarative memory
    Anything that’s fact or knowledge-based falls into this category. Examples include remembering the capital of Japan or what your brother’s phone number is. Neuroscience research indicates that both REM and SWS play a role here—the former plays a role in complex, emotionally-charged declarative memory consolidation while SWS affects acquired facts. (1 + 3)

  2. Episodic memory
    As the name suggests, episodic memory encompasses episodes—or events—that took place in your life. Examples include remembering the moment your kids took their first steps or the first time you went skiing.

  3. Semantic memory
    Semantic memory is a type of long-term declarative memory not drawn from personal experience. Examples include remembering the name of a specific car part or what a baby deer is called.     
  4. Instructional/procedural memory
    If you remember “how” to do something, that’s instructional/procedural memory retention, which REM sleep contributes to. Examples include remembering how to ride a bike or change a flat tire.

You also may hear the phrase "working memory," which refers to short-term, temporary recall that impacts in-the-moment decision making and behavior. This isn't as heavily impacted by sleep quality as other memory processes.

Most new memories we acquire every day are forgotten, which is normal (it’d be impossible to remember every single thing). The ones that DO stay with us, however, undergo an encoding process before being consolidated and reactivated—two processes that, according to new research published in the Annual Review of Psychology, depend heavily on sleep. (4)

  • Memory consolidation
    After you acquire the memory, consolidation affects how it’s stored. The information is stabilized and integrated within the structure of multiple prior memories.

  • Memory recall/reactivation
    This is when information stored in your brain network from a previous experience re-emerges with or without the conscious experience of actually retrieving it.  

Although the exact science behind the consolidation of memory during sleep is unclear, emerging research—including research using functional MRIs (fMRI)—indicates that it has to do with the activation of neural synaptic connections in the brain’s hippocampus and neocortex. In short, the hippocampus transfers newly acquired information into the neocortex. From there, the neocortex gets to work reviewing and processing the memories to help sustain them for the long-term. (5)

“We postulate here that consolidation largely transpires without us knowing about it—because we are asleep at the time,” write researchers. “Moreover, understanding the neurophysiology of memory processing during sleep may be the key to understanding how the memories formed while we are awake are preserved and transformed, and how they transform us. Ultimately, changes in memory storage during sleep may shape not only what we can remember but also who we are.”

Sleep apnea is marked by temporary pauses in breathing throughout the night. Unlike the less common central sleep apnea, which occurs when the brain fails to send correct signals to the muscles in charge of breathing, OSA causes throat muscles to relax, partially block the airway, and lead to snoring as well as lower oxygen levels in your blood. It’s particularly common among menopausal women—about 20% develop some form of the condition. (6) What makes OSA so tricky is that many people don’t know they have it—for some, the only symptom is unexplained fatigue, while others may experience sleep disruptions due to snoring or a feeling of gasping for air.

Memory consolidation is a function of sleep, which explains why OSA (with the fragmented sleep periods, disrupted sleep cycles, and bouts of wakefulness that it causes) doesn’t bode well for memory performance. According to a study published in the Journal of the International Neuropsychological Society, OSA correlates with semantic memory impairment. (7)

“This study demonstrated a deficit in semantic autobiographical memory recall and in the ability to retrieve specific memories from one’s past in individuals with untreated OSA. These results point toward the likelihood that OSA impairs the capacity to either encode or consolidate semantic autobiographical memories, leading to difficulties with the recall of specific details from the past. Poor autobiographical memory is a risk factor for developing persistent depression, which is a significant disease burden in the OSA population.”

READ MORE: When To See A Sleep Specialist: 6 Signs + Symptoms

In order to understand the effects of sleep on long-term memory function and Alzheimer’s risk in older adults, we have to talk about beta-amyloid. This protein lives in the fluid between our brain’s cells or neurons. A build-up of beta-amyloid plaques has been linked to impaired brain function and Alzheimer’s disease, but there is something we can do to help clear it out: sleep.

In a recent study conducted by the National Institutes of Health, researchers scanned participants’ brains after a night of sleep. (8) The study group was sleep deprived (defined as going 31 hours without sleep), while the control group got a complete night’s rest. The results showed that levels of beta-amyloid increased about 5% for the sleep-deprived group specifically in the thalamus and hippocampus, both of which are susceptible to damage in early Alzheimer’s.

Interested in learning more about the science of sleep? We've got you covered.