How Sleep Helps Us Remember and Forget

Post by Amanda McFarlan

What’s the deal with sleep?

Humans spend approximately one third of their lives sleeping, so it is no surprise that we’re curious about it! Sleep has a wide variety of benefits, like repairing and regenerating tissues in the body, improving cognitive and physical performance, and consolidating memories. On the other hand, a chronic lack of sleep can put us at risk of developing health problems like cardiovascular disease, high blood pressure, diabetes, and depression. So, what happens when we sleep? Every night, when our heads hit the pillow, we enter into the first stage of ‘non-Rapid Eye Movement’ (non-REM) sleep. Non-REM sleep consists of 4 stages, with Stage 1 being the lightest sleep stage and Stage 4 being the deepest. Your body moves through the 4 stages of non-REM sleep and finally through REM sleep in a cycle that takes approximately 90 minutes, and this cycle is repeated throughout the night. Non-REM and REM sleep are characterized by different brain activity patterns, with non-REM sleep creating slow waves in its deepest stages, called ‘slow-wave sleep’, and REM sleep generating activity patterns that resemble wakefulness. The role of non-REM and REM sleep in the transfer and long-term storage of memories, known as memory consolidation, has been studied for many years. Here, we will discuss how sleep helps us remember or forget, as well as what goes wrong when we don’t sleep.

How does sleep help us remember?

Evidence strongly suggests that sleep is integral to memory consolidation. For example, a behavioural study, in which participants performed a visual task, a motor sequence task, and a motor adaptation task, found that participants’ performance was greatly improved if they had a full night’s sleep compared to those that did not sleep. The degree of performance improvement for each type of task was dependent on improved sleep in different stages in the sleep cycle. These findings suggest that non-REM and REM sleep both play an important role in memory consolidation. In line with this, other studies have shown that intensive learning of a new task is followed by increased time spent in REM sleep, resulting in subsequent task improvement, as well as the amplification of slow waves during non-REM sleep. Sleep results in a reactivation of cells in the hippocampus, which subsequently reactivate representations of memory in the cortex, also known as an engram. Over time, after many reactivations, these memories become distributed and consolidated within the cortex. 

Interestingly, research has shown that while we’re sleeping there is increased activity in the same hippocampal place cells (neurons that are activated when moving through specific locations in the environment) that were active throughout the day. This reactivation of hippocampal place cells during REM sleep follows a theta frequency band pattern of firing, hypothesized to be critical for memory consolidation. This hippocampal activity is mediated by neurons that release the neurotransmitter acetylcholine in the hippocampus. Acetylcholine, which plays a major role in altering the strength of synaptic connections, crucial for memory, is known to be elevated during REM sleep. REM sleep has also been associated with the upregulation of the expression of several calcium-dependent genes that are thought to be involved in synaptic plasticity and memory consolidation. 

Compared to REM sleep, the conditions in non-REM sleep are less ideal for promoting synaptic plasticity. For example, acetylcholine and calcium-dependent genes are expressed at low levels or are absent altogether during non-REM sleep. However, researchers have proposed that non-REM sleep might be important for the later stages of memory consolidation, rather than the initial conversion of short-term memories to long-term memories. In support of this, protein synthesis, which is required for long-term but not short-term potentiation (strengthening) of synapses, is increased during non-REM sleep. Therefore, the induction of protein synthesis during non-REM sleep may act to strengthen the synapses that were sufficiently potentiated during wakefulness. 

Although the majority of research on sleep and memory focuses on the role of the hippocampus in memory consolidation, a recent study has provided evidence that the thalamus might also play a role in memory consolidation during sleep. In this study, memory encoding (when memories are initially stored) during a visual task was shown to increase the activity of sensory relay nuclei of the thalamus in mice. Following a night of sleep, the primary visual cortex also showed evidence of a potentiated response to the visual task. Together, these findings suggest that task-related information may be passed from the thalamus to the primary visual cortex, resulting in the formation of a corresponding memory during sleep.

How does sleep help us forget?

Sleep research is centered around how we remember. However, sleep arguably plays just as important a role in the process of forgetting memories. The hippocampus serves as a temporary storage area for newly formed memories until they can be consolidated and integrated into long-term memory storage in the cortex. As a result, the hippocampus must be able to unlearn memories that have already been consolidated or memories that are not pertinent in order to store new memories. Research has shown that in addition to helping with memory consolidation, sleep is also important for unlearning memories. Studies in rats have shown that following sleep, there are widespread reductions in dendritic spines (protrusions on the dendrite that form synapses with nearby neurons) in the cortex as well as a reduction in receptors on glutamatergic neurons that are critical for memory and learning.

Norepinephrine and serotonin are two neurotransmitters in the brain that are associated with the enhancement of synaptic plasticity. During REM sleep, however, norepinephrine and serotonin signaling is suppressed, suggesting that REM sleep may allow for the depotentiation — or weakening — of synapses.  

What happens when we don’t sleep?

We all know how difficult it is to get through the day after a sleepless night. Suddenly, concentrating on what was previously a trivial task can become very challenging. Neuroimaging data has shown that sleep deprived individuals recruit more brain areas while performing the same cognitive task compared to individuals who slept normally. Moreover, brain imaging studies have revealed that hippocampal function is greatly reduced following one night of sleep deprivation, which suggests that losing sleep may actually disrupt our ability to learn new things. Sleep deprivation studies in rats have demonstrated the importance of REM sleep for learning as well as the induction and maintenance of long-term potentiation of synapses during learning. Additionally, REM sleep deprivation was shown to impair learning-dependent neurogenesis (the formation of new neurons) in the hippocampal dentate gyrus, which can impact future learning. The role of REM sleep for learning and memory is particularly relevant for individuals who are treated for depression with antidepressants, since these medications can greatly reduce the amount of time spent in REM sleep and may potentially have consequences on the efficacy of memory consolidation.

How can we get a good night’s sleep?

Given what we know about the role of sleep for learning and memory, it’s important to ensure that we get a good night’s sleep. However, with the challenges of daily life, this is not always an easy feat. First, it is important to establish a regular sleep schedule where you go to sleep and wake up around the same time each day, even when traveling or on the weekends. This habit can reinforce your body’s circadian rhythms, which helps your body to prepare for sleep and wakefulness more efficiently. Second, it is important to avoid using electronic devices before bed, like watching television or using your phone or tablet. The blue light that is emitted by these devices tricks our bodies into thinking it is daylight, and, as a result, our bodies produce lower levels of the hormone melatonin which promotes sleep. Third, use what you know about the science of sleep cycles to your advantage by timing your sleep in 90-minute intervals. For example, by setting your alarm for 7.5 hours of sleep (5 sleep cycles x 90 minutes each) you may actually feel more refreshed than if you slept for 8.5 hours and were awakened during the middle of a deep stage of sleep. Finally, avoiding caffeine and naps late in the afternoon or evening, as well as avoiding large meals or exercise right before bed may help to promote better sleep. 

Now, time to consolidate all of this learning with a good night’s sleep!

Feld, G.B., & Born, J. Sculpting memory during sleep: concurrent consolidation and forgetting. Current opinion in neurobiology, 44, 20–27 (2017). https://doi.org/10.1016/j.conb.2017.02.012

Klinzing, J.G., Niethard, N. & Born, J. Mechanisms of systems memory consolidation during sleep. Nat Neurosci 22, 1598–1610 (2019). https://doi.org/10.1038/s41593-019-0467-3

Poe, G. R., Walsh, C. M., & Bjorness, T. E. Cognitive neuroscience of sleep. Progress in brain research, 185, 1–19 (2010). https://doi.org/10.1016/B978-0-444-53702-7.00001-4

Stickgold, R. Sleep-dependent memory consolidation. Nature 437, 1272–1278 (2005). https://doi.org/10.1038/nature04286