New research suggests that ill-timed activity between two types of brain wave during deep sleep could be the reason that older people sometimes fail to remember that which happened only yesterday.
Researchers from the University of California, Berkeley (UC Berkeley) have discovered that aging disrupts the "coupling" between slow waves and "sleep spindles."
This coupling is important for consolidating new memories during non-rapid eye movement (non-REM) sleep.
"The mistiming," explains senior study author Matthew Walker, a professor of neuroscience and psychology, "prevents older people from being able to effectively hit the save button on new memories, leading to overnight forgetting rather than remembering."
A report on the research that was recently published in the journal Neuron also provides evidence that this age-related deficit is likely due to deterioration in a part of the brain that helps us to enjoy a good night's sleep.
The importance of sleep
We spend around a third of our lives asleep. And as we slumber, our brains create and maintain the circuitry that allows us to make new memories, learn, concentrate, and respond to the world around us.
There are two basic types of sleep: rapid eye movement (REM) sleep and non-REM sleep, each of which has a specific pattern of brain wave and brain cell activity.
Non-REM sleep has three stages: stage 1 is a short period during which wakefulness moves into sleep; stage 2 is when light sleep moves into deeper sleep; and stage 3 is the deep sleep that refreshes us.
Typically, we go through all the stages of non-REM and REM sleep several times during a night's sleep. Research has suggested that both non-REM and REM sleep are important for memory consolidation and that we spend less time in REM sleep as we age.
The team was interested in the interaction between two types of brain wave that occur during non-REM sleep: slow brain waves and short bursts of electrical activity called "spindles," which get their name from the pattern that they make on electroencephalographic (EEG) readouts.
Previous studies have suggested that the "coupled interaction" between slow waves and spindles during non-REM sleep is important for consolidating new memories, but there is "little evidence in humans" to support this, note the authors.
Slow waves and spindles need to be in sync
For their study, the researchers recruited "cognitively normal" younger and older adults and asked them to complete a memory task. They then compared their ability to remember after a night's sleep. As the participants slept, EEG machines recorded their brain activity.
The younger subjects were in their 20s and the majority of the older ones were in their 70s. Both groups were asked to memorize word pairs and complete tests of their recall after a night's sleep. As they did the recall tests, the researchers scanned their brains using functional and structural MRI.
Compared with the younger adults' night-time EEG readings, those of the older adults showed a consistent and early peaking of spindle activity that was not well-coordinated with the slow brain waves.
Prof. Walker — who is also director of the Center for Human Sleep Science at UC Berkeley — likens the process to tennis players coordinating their movements as they do serves. First, they throw up the ball (the slow wave), and then they must hit it at just the right time (the spindle).
There is only a very narrow time window in which to hit the ball to ensure an ace. And in memory consolidation, the scientists say that the brain's time window is about a tenth of a second.
If the brain misses this window of opportunity, it misses the chance to put information held in short-term memory into long-term memory. This happens more often in older brains.
"As the brain ages," explains Prof. Walker, "it cannot precisely coordinate these two deep sleep brain waves. Like a tennis player who is off their game, they're swiping and missing."
Frontal lobe atrophy is the likely cause
A comparison of the MRI brain scans also revealed that the older adults had greater deterioration in gray matter in the medial frontal cortex, compared with the younger adults.
The researchers suggest that such "atrophy" in this part of the brain's frontal lobe is the likely cause of the uncoupling of spindles and slow brains waves.
"The worse the atrophy in this brain region of older adults," Prof. Walker explains, "the more uncoordinated and poorly timed are their deep sleep brain waves."
However, he also notes that the "silver lining" is that this result points to sleep as a potential target for treating memory problems in older people.
He and his team are already planning further experiments. They want to find out whether boosting the slow waves by electrically stimulating the frontal lobe of the brain can get them back in sync with the spindles.
"By electrically boosting these night-time brain waves, we hope to restore some degree of healthy deep sleep in the elderly and those with dementia, and in doing so, salvage aspects of their learning and memory."
Prof. Matthew Walker