The exploration of sleep’s influence over memory has captivated and intrigued scientists for well more than a century. For all the attention paid to the study of sleep and memory, there’s a great deal we don’t yet understand about how these two essential processes interact. As the science of memory function continues to be explored and unraveled, our understanding of sleep’s importance continues to grow. It’s now widely accepted that sleep is far more than a passive contributor to memory function, but plays a very active role in the formation and preservation of memory. There are several different theories currently being investigated as to how sleep affects memory, particularly for the role of sleep in memory consolidation or how memories are put into long-term storage. Nearly all working theories involve sleep affecting memory consolidation across multiple stages of sleep—including stage 2 sleep, deep sleep, and REM sleep—and sleep having a significant impact on memory of all kinds.
Memory can be divided broadly into two categories: declarative memory and procedural memory. Nearly all of what we think of when we think about memory falls within these two categories from skills like walking that we do without thinking to the conscious memories we create and store about our lives. Sleep affects both declarative and procedural memory, and in so doing casts a shadow that is both broad and deep over our ability to retain and recall information that is of meaning and significance to us, as well as information that is essential to how—and how well—we live out our lives.
Declarative memory involves memories of facts and knowledge that we consciously recall as well as details about our individual lives. Declarative memory is also known as explicit memory because we can intentionally seek to both acquire and remember these memories. Declarative memories can be deeply personal and emotional when they include details and emotions we may have felt during a challenging part of our life. They also include the knowledge we gain throughout life. The creation of declarative memories involves the brain’s middle portion of the brain’s frontal lobe where consciousness probably lives along with problem solving ability, complex thinking and planning, impulse control, and emotional regulation. Declarative memories are first are created here and stored temporarily until they can be consolidated in a process that we now believe depends on sleep.
There’s a growing body of research that indicates sleep is critical to the making and storing of declarative memory. Slow-wave sleep in particular appears to be especially important to declarative memory. Many studies suggest that during slow-wave sleep—also known as deep sleep—the declarative memories made during the day are reactivated and redistributed to other brain networks for long-term storage. This function starts in the earliest stages of life. Scientists studying memory processing in infants found that babies 6 to 12 months who took naps at least 30 minutes after learning new behaviors showed better recall of those behaviors than infants who did not sleep.
Studies also show that getting insufficient sleep can negatively affect declarative memory storage. People with obstructive sleep apnea don’t sleep as well and rarely get into deep sleep. As a result, they seem to have substantially lower next-day recall for declarative memory tasks, compared to people without sleep apnea. People who suffer from insomnia also have showed diminished slow-wave sleep and a corresponding decrease in consolidation of declarative memories.
Procedural memories are task and skill-based memories tied to movement and sensation. Much of the basic knowledge we need to function on a daily basis from tying shoes to driving a car to taking a run at the gym falls within the category of procedural memory. Unlike declarative memory, procedural memory is often recalled unconsciously and automatically and is generally made through repetition and practice. Hurdlers running on a track or musicians playing guitar, for example are developing and relying on procedural memory. Because the input for these memories is different, the brain uses different areas to remember them.
In spite of these differences, procedural and declarative memories both require sleep to function normally and well. A routine of high-quality, plentiful sleep is important for motor-skill learning and procedural memory. One recent study looked at the influence of sleep over motor memory by asking participants to learn a finger sequence similar to playing a piano. One group learned the finger sequence at night and then slept shortly thereafter. This group was tested on their memory for the sequence the next morning. The other group learned the finger sequence in the morning and stayed awake throughout the day, before being tested on the finger sequence at night. Those who slept had heightened levels of activity in areas of the brain involved in motor skill consolidation and greater evidence of communication among neural networks. The non-sleeping group, on the other hand, showed a reduction in brain activity over the period between learning and recall.
This study highlights how effective it may be for memory recall to sleep relatively soon after learning takes place. A number of studies indicate that sleeping shortly after learning enhances and improves memory formation, for both declarative and procedural memories. In real-world terms, this means that burning the midnight oil isn’t actually a smart strategy for being more productive or gaining an edge either on the field, in the classroom, or at work. It also means that naps may in fact be a wise investment in an otherwise busy, task-filled day.