Sleep is probably the most unrecognized, yet, one of the most efficient studying methods. Sleep is essential for memory because memory traces are created and strengthened during sleeping hours. Hippocampus is a brain area that is associated with consolidating memory sleeping hours.
Memories need time to consolidate
There are two processes in learning information and storing it into long-term memory: 1. acquisition and 2.consolidation.
In acquisition, information is learned for the first time and processed in the working memory. This is a form of “online processing” of the information (happening here-and-now). Working memory processes happen at the front parts of the brain (prefrontal cortex or PFC which is located just behind and above the forehead).
Information consolidates during rest and sleep. During consolidation, new information gets integrated with old knowledge. This happens in so-called “offline hours”, i.e. when the person is concentrating on other activities. Most of it happens during sleeping. This process is linked to the function of two brain areas: hippocampus and neocortex.(S) Hippocampus is like a bridge between the “online”-memory processes and “offline”-memory storage, which means that binding new information to old knowledge during sleep takes place in the hippocampus. The brain area itself sits deep in the brain below the ears. Hippocampus is also activated information is “picked up” later from the neocortex back to working memory. (S)(S)
What science reveals about sleep and memory
It has been shown that storing information to long-term memory happens mostly during sleep. The typical way to test this is by comparing two groups of people who both learn information (e.g. word-pairs, word lists or motor skills). One group is the “Sleep group” and uses the study-sleep-study technique. They learn information in the evening (for example, 9 pm) and are tested in the morning (for example 9 am). No-sleep group learns information in the morning (9 am) and is tested in the evening (9 pm). That also leaves 12 hours between learning and the test. Nothing else between these groups changes; the testing interval is exactly the same and no artificial sleep-deprivation is caused. An extensive review of the studies has been offered by sleep researcher Matthew Walker in his book Why we sleep. Here are some research highlights:
1. Sleep improves motor accuracy and speed
In this study, two groups were learning fine motor skills, specifically finger tapping. The sleep group had a 20 % better motor speed and accuracy than the no-sleep group (S). Other studies have confirmed these findings (S).
2. Sleep improves memory for facts
If there is a need to memorize a lot of factual information (such as in education) sleeping will help to “stick” the information in the brain and prepare the brain to learn more.
In this study (S), groups studied word pairs, where the words were related to each other (easy to learn) or non-related to each other (harder to learn)
Ellenbogen, J. et al. (2006).
Sleep-group’s recall was 95 %
No-sleep group’s recall was 80 %
3. Sleep prevents interference of other information (S)
This study is the second part of the previously mentioned word-pair study. The setting was the same, only this time both groups had to study a second list of words (interference list) just before the test. Thus, the sleep-group studied the second list after a night sleep (in the morning) and no-sleep group studied it on the same day (in the evening). Now the recall for the original words dropped, but the difference between sleep and the no-sleep group was huge (see the graph above)
Sleep group’s memory for the original words declined 15 %-points
Non-sleep group’s memory for the original words declined 40 %-points
4. Sleeping before learning improves the potential to learn (S)
Learning is more efficient after a good night’s sleep. In other words, sleeping prepares the brain for subsequent learning.
In one study college students were assigned into two groups: sleep-group and sleep-deprivation group (for 36 hours). After that, students had to learn a list of facts inside a brain scanner.
The sleep-deprivation group learned 40 % less information compared to the group who slept before learning.
The sleep group had healthy and beautiful hippocampal activity while they were learning meanwhile the sleep-deprivation group showed almost no hippocampal activity at all.
In other words, staying awake all night will lessen the ability to learn information on the following day.
5. Sleep increases memory for positive information (S)
In addition, in one setting students were learning either positive or negative valenced words.
The sleep-deprived group showed better recall for the negative information than the positive one (so-called negativity bias, which is observed for example in depressed individuals)
The sleep group recalled twice as much positive information than the sleep-deprived group.
Thus, a good night’s sleep will not only improve memory but make life a little bit more positive.
Why sleep improves memory?
Brain scans show that during sleep hippocampus is active and makes the brain “train” the information. Actually, brain activity in certain sleep stages resembles the same activity observed in the moment of learning. (S) (S)
There is some evidence for specific sleep stages for a specific type of memory. Yet, we should keep in mind that this is really a line drawn in water and there is still a lot more to learn about the sleep stages and memory.
Current evidence links especially REM sleep to motor skills training. REM is also linked to a brain process called neuroplasticity, which means that REM is important in causing brain structure changes. This happens because, during the REM-sleep stage, the genes that help neuroplastic changes to happen are more active. Studies have also found that selectively inhibiting only REM sleep and stage 2 sleep, causes impairments in motor learning (S) However, as deep sleep is also important for muscle recovery and the post-exercise muscle growth, and also helps building and strengthening the neuromuscular connections, both sleep stages are important for motor skills training. (S) (S)
Deep Sleep. Deep sleep has shown to be especially important for learning facts. It has been shown that during the early hours of sleep (deep sleep) there are more sleep spindles in response to more learning. (S) (S).
How poor sleep can cause Alzheimer’s disease
Human studies show that sleep deprivation, especially deep sleep deprivation, increases toxins in the brain and this can cause Alzheimer’s disease.
One of the reasons for Alzheimer’s disease is the accumulation of a harmful substance called beta-amyloid in the hippocampus. Deep sleep deprivation can increase the risk of Alzheimer’s. This was demonstrated in one study were people were deprived of deep sleep and the next morning they had 20 % more beta-amyloid circulating in their cerebrospinal fluid (S)
This happens because during deep sleep brain detoxifies with the help of the glymphatic system (i.e. the lymphatic system operated by the glial cells in the brain). The intriguing and astonishing process that detoxes the brain is the shrinkage of brain cells, called glial cells, during the night. Glial cells occupy the space between other brain cells, neurons, which are relaying messages (such as memories and motor actions!). When glial cells shrink during sleep, they create even 60 % more space to the brain, which allows more detoxifying fluids between brain cells to flow (S). This fluid has shown to flush the toxins (such as beta-amyloid) away from the brain during the night and help prevent the accumulation of the substances that can cause memory diseases (S).
In summary, the role of sleep in preventing Alzheimer’s and improving memory might be partially due to the detoxification of the brain during the night.
Sleep consolidates memories and prevents new information from interfering with new information. Thus, helps to prevent forgetting and saves time from re-learning information.
Evidence shows that sleep is beneficial both before learning and after the learning. Before learning sleep can improve the ability to take in information even by 60 %. After learning, the night-time hippocampal activity in the brain consolidates the memories from the day.
Brain scans show that during sleep the brain is training the information that one has learned during the day. That is, its producing similar neural activation than during day-time learning – all while the conscious mind is happily knocked out. Studies show that the brain can is very efficient in rehearsing information during offline hours, in comparison to staying awake and trying to cram the information to the brain.
Approximately 8 hours of sleep each night is needed to get enough REM and deep sleep and improve memory performance. But really, it should not be about needing as much as deserving to sleep. Sleeping s fun, it’s healthy and it’s an absolutely efficient way to consolidate memories.
In addition, sleep deprivation can increase negativity bias, and sleep improved memory for positive information.
Top tips based on the research
- If you can, always get 8 hours of sleep at night
- Study prior to bed in order to improve memory
- Sleep in between study periods in order to prevent memories to confuse and get forgotten (study-sleep-study)
- Remember that the brain needs time to consolidate new information: sometimes you need to wait days or even weeks for really imprint new memories in your brain (rehearsal speeds up this process)
- Sleep well especially during professional training or exam weeks, when new information is presented to you in a fast phase
- Don’t pull an all nigher -> you can reduce your potential to learn even by 40 %!
- Sleep is important for both motor and factual learning
- Remember that after you have learned, you deserve sleep; make sleep your reward rather than a necessity
- If you want to improve your sleep, read my 30 powerful tips for getting 8 hours of sleep every night
Bird, C. M., & Burgess, N. (2008). The hippocampus and memory: insights from spatial processing. Nature Reviews. Neuroscience, 9(3), 182–194. https://doi.org/10.1038/nrn2335
BREM, A.-K., RAN, K., & PASCUAL-LEONE, A. (2013). Learning and memory. Handbook of Clinical Neurology, 116, 693–737. https://doi.org/10.1016/B978-0-444-53497-2.00055-3
Interfering with theories of sleep and memory: sleep, declarative memory, and associative interference. – PubMed – NCBI. (n.d.). Retrieved May 13, 2019, from https://www.ncbi.nlm.nih.gov/pubmed/16824917
Ju, Y.-E. S., Ooms, S. J., Sutphen, C., Macauley, S. L., Zangrilli, M. A., Jerome, G., … Holtzman, D. M. (2017). Slow wave sleep disruption increases cerebrospinal fluid amyloid-β levels. Brain, 140(8), 2104–2111. https://doi.org/10.1093/brain/awx148
Kuriyama, K., Stickgold, R., & Walker, M. P. (2004). Sleep-dependent learning and motor-skill complexity. Learning & Memory, 11(6), 705–713. https://doi.org/10.1101/lm.76304
Memory for Semantically Related and Unrelated Declarative Information: The Benefit of Sleep, the Cost of Wake. (n.d.). Retrieved May 13, 2019, from https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0033079
Rasch, B., & Born, J. (2013). About Sleep’s Role in Memory. Physiological Reviews, 93(2), 681–766. https://doi.org/10.1152/physrev.00032.2012
Smith, null, & MacNeill, null. (1994). Impaired motor memory for a pursuit rotor task following Stage 2 sleep loss in college students. Journal of Sleep Research, 3(4), 206–213.
Walker, M. P., Brakefield, T., Morgan, A., Hobson, J. A., & Stickgold, R. (2002). Practice with sleep makes perfect: sleep-dependent motor skill learning. Neuron, 35(1), 205–211.
Walker, M. P., & Stickgold, R. (2006). Sleep, memory, and plasticity. Annual Review of Psychology, 57, 139–166. https://doi.org/10.1146/annurev.psych.56.091103.070307
Wible, C. G. (2013). Hippocampal Physiology, Structure and Function and the Neuroscience of Schizophrenia: A Unified Account of Declarative Memory Deficits, Working Memory Deficits and Schizophrenic Symptoms. Behavioral Sciences, 3(2), 298–315. https://doi.org/10.3390/bs3020298
Xie, L., Kang, H., Xu, Q., Chen, M. J., Liao, Y., Thiyagarajan, M., … Nedergaard, M. (2013). Sleep drives metabolite clearance from the adult brain. Science (New York, N.Y.), 342(6156), 373–377. https://doi.org/10.1126/science.1241224