Common sense holds a well-rested person can learn more easily and more thoroughly than a sleepy person. Let’s look at the science.
Rested brains learn more readily. Well-rested (not sleep- deprived) brains do a bunch of tasks better than sleepy brains. In tests of response time to stimuli, agility, ability to remember new material and to perform things like mental arithmetic, the superiority of the rested brain has been shown again and again. It’s so cliché that these types of tests are performed by psychology students early in their training in order to get their feet wet with research.
A more interesting finding is how sleep after learning something helps cement the learning. During sleep the brain turns recently acquired memories into long term memories. Sleep helps lock in the learning. This appears to be one of the main biological functions of sleep.
A breakdown of learning can be Acquisition, Consolidation, and Recall. Acquisition and Recall occur during waking (and happen better when the person is not sleepy.)
Consolidation may happen to some extent during waking, but light sleep (stage 2) is the go-to place for consolidation of memories, "making sense" of them, and integrating them into long-term memory.
Acquisition refers to the introduction of new information into the brain. Consolidation represents the processes by which a memory becomes stable.
Now if you want to look at the effect of sleep on learning, three major areas of inquiry come to mind:
This model does not discriminate between declarative memory and procedural memory.
Sleepy people are less vigilant; they have less mental energy and capacity for attention; they tend to be in a poor mood. None of this is conducive to learning.
Recall shares mental mechanisms with other cognitive and physical skills. If your motor performance declines when sleepy, it is reasonable that your recall should also.
EEG readings of stage 2 sleep show short bursts that sleep scientists have named Spindles (because of how they look on the EEG record.) Each Spindle lasts about a second, and there may be as many as a thousand in each person’s brain every night. Spindle events are not understood by neuroscientists. It is suspected they have something to do with the transfer of memory from the hippocampus to the frontal cortex and the formation of long-term memories. Sleep spindle activity is associated with the integration of new memories with existing knowledge. The number of spindle events declines in old age, which is thought to correspond to overall decline in mental acuity in older people.
There may not be a biomarker for sleepiness, but sleep spindle activity could serve as a physiological indicator of a type of intelligence.
The number and density of the spindles as displayed on an EEG more or less correlates with mental activity we call intelligence and these spindles increase during periods of learning.
The light sleep of stages 1 and 2 have been shown to be important in helping the brain being plastic to learning new material. This also points to an explanation for why a 90-minute nap in the day can help people remember material they just learned. And why "sleeping on it" often helps people mentally digest their situation.
In a normal night's sleep, the majority of the stage 1 and 2 sleep happens in the second half of the night. If you cut your sleep short, you may not learn as much as you could from the previous day's events. Although it has long been known that a period of sleep assists in solving problems, new research found that the effect is greater for difficult problems than for easy problems.
Skeptics point out that people with high intelligence quotients or good at school do not necessarily sleep more than others.
Sleep is particularly important in learning higher-order abstract concepts. Research has found a significant correlation between the level of improvement in tests of learning and the amount of slow-wave sleep obtained. People consolidate the new learning much better after a period of sleep than during a waking day. Even an afternoon nap helps. More on sleep and memory.
Slow wave (deep) sleep promotes episodic declarative memory consolidation. This is the more important type of memory for schoolwork, in contrast with procedural memory which is important for physical actions. Daytime naps are particularly useful for consolidating new procedural memories. Indeed, electroencephalogram tests of nappers find the more spindle events produced, the better the napper is equipped to learn.
The decline in the amount of time spent in slow wave sleep among old people may explain why older people have a harder time learning new things that younger people. It is indeed hard to teach an old dog new tricks, partly because the dog’s sleep patterns have changed.
Although REM sleep does not appear to play a major part in assimilation and consolidation of new memories, it helps groove the memories by replaying them over in the sleep consciousness. REM also seems to help the person extract generalizations from specific memories, and so if the memories are false, the lessons learned and wisdom acquired can be flawed. REM helps with the higher order mental functions that make us human.
Abstraction is facilitated by REM. One thing that separates humans from animals if our facility at combining temporally and conceptually distinct memories. Stage 2 may be the critical time from acquiring those memories, but REM is where they are combined and higher order abstractions.
Assimilating new memories into old ones and views of the world also occurs during REM. If you are familiar with the concept of Bayesian reasoning, you can sort of thinking as REM as the time when our understanding of the world is updated with new information. So in this hypothesis:
1) In stage 2 the day's experiences are transferred from short-term to long-term memory. The brain hosts a dialogue between the hypothalamus and cerebral cortex in Stage 2.
2) In REM the episode of sleep is integrated into other memories, the view of the world, and high level abstractions. This may take several cycles of REM and perhaps several nights. If the memory is very strong, the person may live it over and over again in their dreams. This can be a problem for people with PTSD.
People with intellectual disabilities get sleep disorders at the same rate as the general population and doctors treat disorders in the same way. Here's an abstract of academic Sleep studies of adults with severe or profound mental retardation and epilepsy
The main difference is that treatment with cognitive behavioral therapy (CBT) may be more difficult, or not feasible. A low percentage of insomniacs are actually treated with CBT anyway.
Here is a good article:
Sleep Disorders in Persons with Mental Retardation - A Significant Factor in Many Behavioral/ Psychiatric Problems? http://thenadd.org/modal/bulletins/v1n5a2~.htm
A Northwestern University study found a 90-minute nap helped people learn a new tune. The 90-minute time was considered important by the researchers because the brain goes into slow-wave sleep. The scientists conducting this study played the tune during the slow-wave sleep period.
An example of hypnopaedia? Not exactly. This was a situation where the sleep appeared to help consolidate the thing that was learned during waking and essentially practicing a new skill during sleep. Performing a tune just learned was considered learning a skill, rather than just remembering information.