Sleep is an underestimated entity in the aging process. Understanding the need of sleep as an age supporter there is a need to review the different phases and implications of it on health.
Brain activity is measured in waves lengths with each of them a specific name and function.
Different stages in sleep
Sleep is divided into different phases. There are 4 stages and the REM sleep. All these stages progress in a cycle from stage 1-4 ( also called NREM = Non-Rapid Eye Movement stage, divided in N1 – N4) and end with REM to start over again.
REM = Rapid Eye Movement sleep. During REM the eyes start moving fast and the brain activity is speeding up at different levels. When entering REM sleep, breathing becomes faster, irregular, and shallow, the eyes jerk rapidly in various directions, and the limb muscles become temporarily paralyzed. The heart rate increases, blood pressure rises, and males develop penile erections, females can have a more vaginal extraction. When people awaken during REM sleep, they often describe bizarre and illogical stories called dreams.
The first REM sleep period usually occurs about 70 minutes after falling asleep. A complete sleep cycle takes 90 to 110 minutes on average. The first sleep cycle each night contain relatively short REM period and a long period of deep sleep (Phase 4). As the night progresses, REM sleep periods increase in length while deep sleep decreases. By morning, all time is spent in stages 1, 2, and REM.
A person awakened after sleeping just more than a few minutes is usually unable to recall the last minutes before falling asleep. This sleep-related form of amnesia is the reason people often forget phone calls or conversations from the middle of the night. It also explains why it is possible not to remember the alarms ringing in the morning if going back to sleep after turning it off. During the aging process, this events become more frequent and take a long time for recuperation.
Since sleep and wakefulness are influenced by different neurotransmitter signals in the brain (serotonin and norepinephrine), foods and medicines that change the balance of these signals affect whether people feel alert or drowsy and how well they sleep. Caffeinated drinks such as coffee and drugs stimulate parts of the brain and can cause sleep disorder (even insomnia), or an inability to sleep.
Many antidepressants suppress REM sleep. Heavy smokers often sleep lightly and have reduced amounts of REM sleep. They tend to wake up after 3 or 4 hours of sleep due to nicotine withdrawal in the blood. Many people who suffer from sleep disorders try to solve the problem of alcohol – a so-called nightcap. While alcohol does help people fall into light sleep, it also takes away their ability to enter REM and deeper, the more restorative stages of sleep. Instead, it keeps them in the lighter stages of sleep, from which they can be awakened easily. Alcoholics show a disoriented consciousness in the morning. Sleep deprivation is a strong aging promotor, the effects are clearly visible in physical and mental expression.
Everybody loses some of the ability to regulate the body temperature during REM, temperatures changes in the environment can disrupt this stage of sleep. If REM sleep is disrupted one night, the body doesn’t follow the normal sleep cycle progression the next time it dozes off. In different scientific sleep research projects, they discovered that instead of starting the new cycle, often people slip directly into REM sleep and go through extended periods of REM until finally “catch up” on the sleep cycle. Hereby the answer is “yes” to the question; can people catch up for lost sleep. But the answer is only “yes” when it happens coincidentally and not often.
The effects of REM sleep deprivation (RSD) on neurotrophic factors, specifically nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), were assessed in 2000 by Sie et el. Neurotrophins are proteins found in the brain and periphery that aid in the survival, functioning, and generation of neurons; this is an important element in the synaptic plasticity process, the underlying neurochemical foundation in forming memories. BDNF protein has been shown to be necessary for procedural learning (a form of non-declarative memory). Since procedural learning has also exhibited consolidation and enhancement under REM sleep, it is proposed that the impairment of procedural learning tasks is due to the lack of BDNF proteins in the cerebellum and brainstem during RSD. In regards to NGF, the basal forebrain (production and distribution of AcH in the brain), more specifically the medial septal area, sends cholinergic (excitatory in the hippocampus) and GABAinergic (inhibitory) neurotransmitters through fibers to the hippocampus target cells. These target cells then secrete NGF which plays a key role in the physiological state of the hippocampus and its functions. It has been noted that REM sleep increases the secretion of NGF, therefore it has been proposed that during RSD cholinergic activity decreases leading to a decrease in NGF and impairment in procedural learning.
REM sleep begins in response to signals sent to and from different regions of the brain. Signals are sent to the brain’s cerebral cortex, which is responsible for learning, thinking, and organizing information. Signals are also sent to the spinal cord to shut off movement, creating a temporary inability to move the muscles (“paralysis”) in the arms and legs. Abnormal disruption of this temporary paralysis can cause people to move while they are dreaming.
REM sleep stimulates regions of the brain that are used for learning. Studies have shown that when people are deprived of REM sleep, they are not able to remember what they were taught before going to sleep. Lack of REM sleep has also been linked to certain health conditions, including migraines. This also refers directly to the aging process which accelerates with lack of sleep and even can exhaust a person completely cutting years of a healthy life.
Macroscopic brain systems
Previous research has shown REM sleep to reactivate cortical neural assemblies post-training on a serial reaction time task (SRT), in other words, REM sleep replays the processing that occurred while one learned an implicit task in the previous waking hours. Results of PET scans indicate that bilateral cuneus was significantly more activated during SRT practice as well as post-training REM. In addition, this activation was significantly increased during REM sleep versus the SRT task. This suggests that specific brain regions are specifically engaged in the post-processing of sequential information. This is further supported by the fact that regional CBF (rCBF) during post-training REM sleep is modulated by the level of high-order, but not low-order learning obtained prior to sleep. Therefore, brain regions that take part in a learning process are modulated by both the sequential structure of the learned material (increased activation in cuneus), and the amount of high-order learning (rCBF).
The positive correlation between sleep and memory breaks down with aging. In general, older adults suffer from decreased sleep efficiency. The amount of time and density of REM sleep and SWS decreases with age. Consequently, it is common that the elderly receive no increase in memory after a period of rest. To combat this, donepezil has been tested in healthy elderly patients where it was shown to increase time spent in REM sleep and improve next day memory recall.
Patients with Alzheimer’s disease experience more sleep disruption than the healthy elderly. Studies have shown that in patients with Alzheimer’s disease, there is a decrease in fast spindles. It has also been reported that spindle density the night before a memory test correlate positively with accuracy on an immediate recall task. A positive correlation between time spent in SWS and next day autobiographical memory recall has also been reported in Alzheimer’s patients.
|Sleep spindles are short and intense bursts of neurons firing in sync, occurring in the thalamocortical networks. This peak late in the night and are defining characteristics of stage two sleep. Sleep spindles are thought to aid in information consolidating during sleep and have been shown to increase after training on a motor task.|
The first stage of sleep is characterized by theta waves, which are slower in frequency and greater in amplitude than alpha waves which occur when relaxing. The difference between relaxation and stage 1 sleep is gradual and subtle. During stage 1, light sleep, a person drift in and out of sleep and can be awakened easily. The eyes move slowly and muscle activity slows down. People awakened from stage 1 sleep often remember fragments of visual images. Many also experience sudden muscle contractions often followed by a sensation of falling.
As the sleeper moves to stage 2 sleep theta wave activity continues, interspersed with two other waves. These phenomena, which occur periodically every minute or so, and are defining characteristics of stage 2 sleep, are termed sleep spindles and K complexes. The former is a sudden increase in wave frequency, and the latter is a sudden increase in wave amplitude. Stages 1 and 2 are relatively “light” stages of sleep. In fact, if someone is awoken during one of these stages, he or she will often report no being asleep at all. When entering stage 2 sleep, the eye movements stop and the brain waves (fluctuations of electrical activity that can be measured by EEG) become slower. This is measured by lengthening of the waves with occasionally a bursts of rapid waves (shown as spikes on the graph).