Sleep and study schedules: Both matter for memory

Image Credit: Microsoft Word.

Reference: Huang, S., Deshpande, A., Yeo, S. C., Lo, J. C., Chee, M. W., & Gooley, J. J. (2016). Sleep restriction impairs vocabulary learning when adolescents cram for exams: The need for sleep study. Sleep39(9), 1681-1690.

“Due tomorrow? Do tomorrow.” – almost every student ever

Procrastination is a common trait, especially among students (1). Often, in order to make up for time put-off, many students will sacrifice their sleep in order to cram for an exam at the last minute. However, sacrificing sleep to cram a lot of content is less effective than you might think.

How sleep aids memory
Sleep is important for learning, as it helps us maintain attention (2) and form/store memories (3). While we sleep, the memory center of our brain (the hippocampus) undergoes some major restructuring. Old memories are moved outside of the hippocampus to other regions of the brain, and then these memories are strengthened. This helps clear up space for the formation of new memories (4). Think of it as backing up your cell-phone photos to the cloud: your phone can only hold so many photos, but if you upload them to the cloud and delete them off of your phone, then there’s room for new photos. (But for your brain, you need to do this over many nights; a single upload session isn’t enough.)

So, students who forgo sleep to cram aren’t giving their brain the much-needed opportunity to reorganize and strengthen their memory of class material. They fail to upload their learned material onto the “brain cloud” and make space for other content.

How study spacing aids memory
Additionally, cramming is actually a very poor way to study because time plays a key role in learning. People learn better when content is spaced out over time compared to when it is crammed into a single learning session. In other words, if you only had two hours to study for an exam, it would be better to study for one hour on two separate days, than to study for two hours in one day (5). One of the reasons why studying over two days is better than studying over one day is that the two study sessions are divided by a night of rest, and, as discussed above, the brain reorganizes and strengthens memories during sleep.

The effects of sleep and study spacing
How does sleep and study spacing interact to influence test performance? To answer this question, researchers Sha Huang and colleagues at the Duke-NUS Medical School in Singapore recruited 60 healthy students between the ages of 15-19. Students who regularly slept less than six hours each night were not included in this study.

Leading up to the start of the study, students were asked to spend from 11pm to 8am (9 hours) as time in bed (TIB). Then, the experiment took place over 14 days. During days 1-3, participations were to maintain their 9 hours TIB. On days 4-10, students were randomly assigned to either stay at 9 hours TIB, or switch to 5 hours TIB (sleeping from 1am to 6am each night. During the final days of the study, all students returned to a 9 hour TIB. Finally, on days 11-14, all students returned to a 9-hour TIB schedule.

Schematic of sleep schedule and study spacing. Adapted from Huang et al. 2016.

To measure memory, students practiced a vocabulary learning task on Day 4 of the study (after the last normal night of sleep). All students completed the same learning task. There were four study sessions between day 4 to 7, where students would learn 40 related word pairs on virtual flash cards. For example, on the front of one flash card it may read “pithy”, and on the back of the card it may read “brief”.

These word pairs were presented on one of two schedules: either “spaced” or “massed”. Pairs that were “spaced” were given to the student in a single stack of 20, and students would learn them twice a day over the four study sessions. Pairs that were “massed” were stacked into four groups of 5, and students would practice one stack of pairs 8 times in each session. As this learning task was completed on computers, the amount of time students interacted with the pairs was controlled. At the end, all word pairs – whether spaced or massed – were viewed 8 times for 80 seconds total.

Finally, students were tested on their ability to recall these word pairs. Students would be presented at random with a word they had been studying (example: pithy) and were instructed to type the target word (answer: brief) These tests occurred at the end of each day during the study sessions (days 4-7) as well as day 8 and 12 to examine long-term retention of the pairs.

What did they find?
During the study portion of the experiment (days 4-7), test performance was the worst when students were 5-hours TIB AND words were learned on the “massed” schedule. Long-term retention (i.e., memory for the pairs on days 8 and 12) was the worst when the words were studied “massed” and the students had 5-hours TIB. Interestingly, test performance was similar between both spaced groups, regardless of whether they were 5-hours or 9-hours TIB.

Results from day 12 of the study, representing long-term retention of the word pairs. Getting 5 hours of sleep and learned words on a massed schedule led to the lowest retention rates, whereas learning words on a spaced schedule (regardless of TIB) led to the highest retention rates.

So, what does this mean?
These results emphasize the importance of study schedule and sleep on memory. Overall, memory is better when we space out studying compared to when we cram. Regarding sleep, those who crammed and got less than 5 hours of sleep performed the worse on a test of their memory. Thus, if students must cram, they should still strive to get a full night’s sleep before their exams.

Directions for future research on sleep and studying
All research has limitations, and this study is no different. For example, the researchers used strict guidelines for the number of times student studied, how long they read each word, the time of day, and so on. The results might vary if any number of these factors changed, and thus further research is needed to understand the boundaries of these effects.

Additionally, although the researchers did not observe differences between the 5-hours TIB and 9-hours TIB in the spaced study conditions, this doesn’t mean that the amount of sleep does not interact with spacing in other contexts. Sometimes, when a learning task is too easy, there can be a ceiling effect, where many participants in the study score close to the maximum score (i.e., recall almost all the word pairs). If the researchers had used a more difficult memory test, there may be a greater range of test scores, which could help tease apart if learning from study spacing is impacted by sleep deprivation.

Students: Stop procrastinating! But if you have to, at least get some rest.
Contrary to what you may think, pulling that all-nighter isn’t going to help you on that exam. If you are still going to procrastinate until the day before your exam, study as much as you can but make sure you are well rested. Better yet, start thinking about how you can find an hour or two each day leading up to that exam study. This will help your learning and allow you more time to get that ever-so-precious sleep!

Additional References

(1) Solomon, L. J., & Rothblum, E. D. (1984). Academic procrastination: Frequency and cognitive-behavioral correlates. Journal of Counseling Psychology, 31(4), 503–509.

(2) Lim, J., & Dinges, D. F. (2010). A meta-analysis of the impact of short-term sleep deprivation on cognitive variables. Psychological Bulletin, 136(3), 375.

(3) Potkin, K. T., Bunney, W. E., & Jr. (2012). Sleep improves memory: The effect of sleep on long term memory in early adolescence. PLoS ONE, 7(8), 42191.

(4) Walker, M. P. (2009). The role of slow wave sleep in memory processing. Journal of Clinical Sleep Medicine, 5.

(5) Kang, S. H. K. (2016). Spaced repetition promotes efficient and effective learning: Policy implications for instruction. Policy Insights from the Behavioral and Brain Sciences, 8, 3(1), 12–19.

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