In stage 3, slow brain waves called delta waves (also called Slow wave sleep (SWS) begin to appear, interspersed with smaller, faster waves. At stage 4, the brain produces mainly delta waves. It is difficult to wake someone during stages 3 and 4, cause these phases are considered deep sleep. There is no eye movement or muscle activity. People awakened during deep sleep do not adjust immediately and often feel groggy and disoriented for several minutes after they wake up. It is strongly advised not to wake a person in this period. Some children experience bedwetting, nightmares, or sleepwalking during deep sleep as they lose all control over the body. Slow wave activity is increased by as much as 25% after implicit learning and time spent in this sleep stage has been shown to improve the performance of the implicitly learned task after sleep.
A person under anesthesia or in coma often said to be asleep. However, people in these conditions cannot be awakened and do not produce the complex, active brain wave patterns seen in normal sleep. Instead, the brain waves are very slow and weak, sometimes all but undetectable. After the operation, the waking up causes serious vomiting and an ill feeling. This is not only caused by the use of the anesthesia but also because of a disturbed sleep cycle. Most of the time it is cured within days.
One important aspect of sleep is memory efficiency. The storage of day to day affairs, faces, and meetings, learning, and processing. To compare with something easy to understand; it is a reset of the hard drive of the personal computer using a cleaning program. When the computer is rebooted it is supposed to work well.
| Hormone actions and sleep patterns
Growth hormone is typically secreted in the first few hours after the onset of sleep and generally is released during slow-wave sleep (SWS). Cortisol is tied to the circadian rhythm, and peaks in late afternoon, regardless of the person’s sleep status or the darkness/light cycle. Melatonin is released in the dark and is suppressed by light. Thyroid hormone secretion occurs in the late evening. |
Consolidation
Consolidation of a memory is a process that takes an initially unstable representation and encodes it in a more sturdy, effective and efficient manner. In this new state, the memory is less susceptible to interference. There are essentially three phases of memory consolidation and all are thought to be facilitated by sleep or not sleep:
Reconsolidation
Reconsolidation of a memory involves the retrieval of an already consolidated memory (explicit or implicit), into short-term or working memory. Here it is brought into a labile state where subsequent information can ‘interfere’ with what is currently in memory, therefore altering the memory. This is known as retroactive interference and is a significant issue for court and eyewitness testimonies.
Positron emission tomography (PET) is used in viewing functional processes of the brain (or other body parts). A Positron-emitting radionuclide is injected into the bloodstream and emits gamma rays which are detected by an imaging scanner. Computer analysis then allows for a 3-dimensional reconstruction of the brain region or body part of interest.
Functional magnetic resonance imaging (fMRI) is a type of brain imaging that measures the change of oxygen in the blood due to the activity of neurons. The resulting data can be visualized as a picture of the brain with colored representations of activation
The main method of measuring sleep in humans is polysomnography (PSG). For this method, participants often must come into a lab where researchers can use PSG to measure total sleep time, sleep efficiency, wake after sleep onset, and sleep fragmentation. PSG can monitor various body functions including brain activity (electroencephalography), eye movement (electrooculography), muscle movement (electromyography), and heart rhythm (electrocardiography).
Electroencephalography (EEG) is a procedure that records electrical activity along the scalp. This procedure cannot record activity from individual neurons but instead measures the overall average electrical activity in the brain.
Electrooculography (EOG) measures the difference in electrical potential between the front and the back of the eye. This does not measure a response to individual visual stimuli but instead measures general eye movement.
Electromyography (EMG) is used to records the electrical activity of skeletal muscles. A device called an electromyograph measures the electrical potential of muscle cells to monitor muscle movement.
Electrocardiography (ECG or EKG) measures the electrical depolarization of the heart muscles using various electrodes placed near the chest and limbs. This measure of depolarization can be used to monitor heart rhythm.
Actigraphy is a common and minimally invasive way to measure sleep architecture. Actigraphy has only one method of recording, movement. This movement can be analyzed using different actigraphic programs. As such, an actigraph can often be worn similarly to a watch, or around the waist as a belt. Because it is minimally invasive and relatively inexpensive, this method allows for recordings outside of a lab setting and for many days at a time. But, actigraphy often overestimates sleeps time (de Souza 2003 and Kanady 2011).