We spend approximately ⅓ of our time asleep, yet scientists still have yet to fully understand the true purposes of sleep. Almost all animals sleep, meaning that there must be some benefits to sleeping that caused it to be evolutionarily conserved across species. Traditional theories about sleep stated that its purpose was to conserve energy or to restore the body’s tissues after a day of activity. Now, scientists believe that sleep may be particularly important for the consolidation of our memories, making it a critical process for our brain health.
What Is Sleep?
Sleep appears to be a passive activity, but the brain is actually highly active when we sleep. During a typical night, we pass through four stages of sleep: stages 1, 2, 3, and rapid eye movement (or REM) sleep.
During stage 1 sleep, we are in a light stage of sleep. The period where you feel as though you are drifting in and out of sleep is known as stage 1 sleep. Stage 2 sleep is deeper, with slower brain waves as well as little bursts of brain waves known as sleep spindles. Stage 3 is characterized by the very slow brain waves known as delta waves. During this period of deep sleep, you do not experience muscle activity or eye movement. If something awakes you from deep sleep, you will likely feel very groggy or disoriented. Finally, during REM sleep, breathing becomes more rapid and your eyes move rapidly in different directions. Dreams occur during REM sleep.
In a typical night, you will spend approximate half of your time in stage 2 sleep. About 20% of the time is spent in REM sleep, while the rest is divided between the other sleep stages. A typical person cycles through stages 1, 2, 3, and REM over a period of 90 to 120 minutes. As a result, you will complete several sleep cycles in a night.
Sleep and Memory Consolidation
Memory is a complicated process, and sleep appears to be critical to laying down new memories that can later be pulled up at will. The first memory step is encoding, in which you attend to an experience that is laid down in the brain. Encoding, or memory acquisition, causes activity within the hippocampus, a brain structure located in the medial temporal lobe. This creates a neural “signature” for that particular memory.
When a memory is first formed, however, it is unstable. This unstable memory trace may be quickly forgotten if it is not consolidated. Sleep appears to be critical to this consolidation process, allowing you to retrieve a memory at a later date. Although scientists are not precisely sure how sleep facilitates memory consolidation, a large body of evidence suggests that sleep is very important for later recall. For example, studies of people who are deprived of REM sleep suggest that they do not recall recently learned information as well as those without REM sleep deprivation. Additionally, researchers who taught people to learn nonsense syllables found that the more sleep the people got, the better they remembered the material after a delay.
Neurobiological studies provide direct evidence of the link between sleep and memory consolidation. For example, the hippocampus is strongly activated during sleep that occurs just after learning something new. This suggests that the cells in the hippocampus are continuing to fire, allowing them to strengthen the connections serving that memory. Furthermore, after new learning, neurons undergo a cascade of events that causes cell connections to be remodeled. This process, known as synaptic plasticity, makes it easier for a given connection to be stimulated in the future.
More recent research suggests that different parts of the sleep cycle may be important for consolidation of different types of memories. For instance, slow-wave sleep may be important for consolidating declarative (fact-based) memory, while REM sleep may be important for procedural memory (how to do something). More research is needed to better understand these processes, but available evidence suggests that all parts of the sleep cycle may be important for certain aspects of memory consolidation.
How Sleep Disorders Disrupt Memory Consolidation
If you suffer from poor sleep, you are certainly not alone. The American Sleep Association reports that 50 to 70 million Americans suffer from a sleep disorder. Even more people experience intermittent periods of poor sleep.
In addition to making you feel chronically tired and worn out, a sleep disorder may also affect your memory abilities. Many people with a sleep disorder say that they have trouble thinking clearly. For many people in this situation, memory problems are twofold: first, difficulty paying attention makes it harder to properly encode information. Second, even for information that has been encoded, lack of good quality sleep impairs memory consolidation mechanisms, preventing long-term memories from being stored. Consider the following sleep disorders and their potential effects on memory consolidation:
Sleep apnea is a condition in which a person temporarily stops breathing during the night, causing a blockage of oxygen to the body and brain. This causes you to awaken for a few seconds before drifting back to sleep. If a sleep partner says that you make gasping or choking sounds in your sleep, sleep apnea may be the culprit. Sleep apnea particularly affects slow-wave sleep, meaning that people with sleep apnea have difficulty with consolidation of declarative memories.
Periodic Limb Movement Disorder
Periodic limb movement disorder is characterized by repetitive limb movements during sleep. This often includes the legs jerking, causing a person to wake from sleep. As periodic limb movement disorder occurs throughout the night, it can affect slow-wave and REM sleep. Many patients with this disorder report extreme grogginess through the day and persistent memory problems.
Circadian Rhythm Disorder
Your circadian rhythm is your “internal clock” that tells your body when to become sleepy and when to awaken. People with circadian rhythm disorder have internal clocks that have gone astray. The sleep phase could be delayed, meaning that you get tired later than you should, or it could be advanced, causing you to become tired too early. In people with a healthy circadian rhythm, the early portion of the night contains more slow-wave sleep, while later portions are primarily REM sleep. Thus, dysregulation of circadian rhythms can disrupt the sleep cycle and cause impaired memory consolidation.
Insomnia, a condition in which you cannot get to sleep, can profoundly disrupt memory consolidation. Insomnia that occurs night after night will cause you to incur a “sleep debt,” in which you need significantly more sleep to make up for the lack of sleep you got earlier. Insomnia reduces the total amount of REM sleep a person receives, leading to poor memory consolidation. Indeed, a recent study (Nissen et al., 2010) found impaired procedural memory in patients with insomnia compared to healthy controls.
Sleep Hygiene Tips to Ensure a Good Night of Sleep and to Improve Brain Health
Even for those who do not suffer from a sleep disorder, sleep quality and duration is not always where it should be. Understanding good sleep hygiene practices can help you get a proper night of sleep, which may facilitate memory reconsolidation. Use the following tips to improve sleep quality:
No electronic devices for at least 30 minutes before bed
Establish a bedtime routine, such as taking a bath or drinking non-caffeinated tea
Limit daytime naps to 30 minutes
Avoid drinking caffeine or using nicotine within 2-3 hours of bedtime
Exercise during the day for at least 10-20 minutes
Limit consumption of foods that trigger indigestion
If these behavioral changes do not help you fall asleep or feel rested in the morning, a sleep disorder may be to blame. Receiving a comprehensive evaluation and sleep study can help determine the cause of your sleep problems, allowing your sleep doctor to make tailored recommendations for you.
If you suffer from poor sleep and memory issues, please contact Sound Sleep Helath at (425) 279-7151 today!