Over the years, science has delved into the nature and function of sleep. Although there are plenty of unanswered questions, slowly, slumber is giving up its secrets.
One role that sleep seems to play a part in is the consolidation of memories. And although rapid eye movement (REM) sleep has long been considered important, non-REM (NREM) sleep has gained more interest recently.
If we conclude that sleep is necessary for firming up memories, it stands to reason that broken sleep should have a detrimental effect.
Recently, a group of researchers ran a study investigating whether or not children with disturbed sleep demonstrate impaired memory. They also hunted for any related neural activity.
Sleep-disordered breathing and memory
In the new study, the researchers focused on children with sleep-disordered breathing (SDB), which refers to a range of symptoms involving different degrees of upper airway collapse.
At the milder end of the spectrum is snoring, and at the other is obstructive sleep apnea (OSA), wherein the airways collapse enough to prevent breathing for short periods during sleep.
With OSA, a person can often stop breathing for 20–40 seconds at a time, many times per night — and even many times per hour — without being aware. These pauses in breathing also cause a reduction in blood oxygen levels.
SDB is relatively common. Some estimate that around 1 in 10 children have some level of SDB. And although OSA has been quite well studied in adults, research in children is lacking.
Some studies have shown that SDB can negatively impact children's academic success, but short-term memory deficits in a pediatric population have not consistently been measured. This may be because neuropsychiatric tests rely on short-term memory, whereas academic performance also relies on long-term memory.
Memory can be broken down into three phases: learning the task, or encoding; consolidation; and recall. During the consolidation phase, the memory trace is stabilized, making it more likely to "stick."
In adults, fragmented sleep has been shown to interfere with memory consolidation, but little research has explored this effect in children.
The new study, which has been published in the journal PLOS ONE, set out to see whether specific memory deficits could be measured in children with OSA. Also, the team wanted to understand more about the specific phases of sleep that are disrupted.
Earlier studies showed that NREM Stage 2 (N2) sleep spindles — short bursts of brain activity that occur during NREM sleep — may be important. NREM slow oscillations are also thought to be involved.
The team wanted to know whether or not these two sleep characteristics were disrupted in children with SDB.
For the study, the team recruited 36 children aged 5–9 and split them into three groups: a control group; children that snored; and children with OSA. Each child was evaluated overnight at Boston Children's Hospital Pediatric Sleep Laboratory in Massachusetts.
Before the study, the children were trained and tested on a spatial declarative memory task, which involved turning over cards to find matching pairs. This was similar to the popular game known as "Memory" or "Concentration."
They also carried out a psychomotor vigilance task wherein the participants were asked to press a button whenever a red circle appeared at the center of a black computer screen. This was to measure attention levels before the memory task.
The children were first tested 30 minutes after waking in the laboratory. Then, 1 week later, they were tested at home 30 minutes after waking, and then again 10 hours later.
As expected, the team found that "participants with mild OSA had impaired memory consolidation." This deficit was measured during the tests at the sleep center and also in those carried out at the participants' homes.
They found that the level of deficit correlated with reduced numbers of N2 sleep spindles. They also noted that N2 sleep spindles were reduced more in those who snored the most. However, contrary to their expectations, no relationship was found between NREM slow oscillations and memory consolidation.
The authors conclude:
"[T]his study provides needed evidence that memory processes are disrupted at even low severity levels of SDB and suggests that treatment of mild OSA could be beneficial for memory processes in children."
Although the study was only carried out on a relatively small group of children, the results add to the current understanding of sleep disturbance and its effects on memory.
The researchers believe that in the future, N2 sleep spindles might be a useful way to predict the cognitive effects of SDB in children. And because SDB impacts so many children, understanding potential deficits and designing ways to reverse them are vital.